Rumford's stands first. This consists of a thin, upright rod, with a white cardboard screen a few inches behind it. The candle is placed for ease of reckoning at 10 inches from the screen, the light, whose candle power is required, is placed on one side of the candle, so that there are two shadows produced on the screen; then the brighter light is moved away from the screen until the two shadows are equal. Assuming this distance to be 40 inches, then the intensities of the two lights are inversely as their squares, therefore the brighter light would be equal to 16 candles. Bunsen's photometer consists of a piece of paper, containing near its centre a round grease spot. The two lights are so arranged that the grease spot becomes invisible. I much prefer my own photometer to any I have ever seen or used. It can easily be made, and will, I feel sure, prove itself to be one of the best yet made. I have given a rough sketch of it, so that you can easily make one of them. (A) is I [35716.]-Legal.-The only way to attain the collimator; then get position for second prism; object which "H. L." has in view is to take steps before taking off the board pencil round the posito have his father placed under the care of the tions; then mark out on the base to be used; then Lunacy Office and a committee of his estate fix collimator and two prisms, and build telescope appointed. The Court will then make orders for on a movable arc, with about 30° of motion.-W. the management of his estate as they think J. LANCASTER. desirable.--SPERANS, Lincoln's Inn. [35723.]-Electro-Coppering Iron.-The blue [35717.]-Lathe Castings. In order to make colour is produced by the copper being in solution sessed of a good one already, else how could you bath. The green is probably a coating of verdia lathe it would be indispensable that you be pos-in an alkali, and will not affect the working of the make the mandrel, bearings, pulley, moving head-gris, and will dissolve off. Why dont you use the advisable, if you have the temerity to attack such you would find it more effective than your alkaline stocks, also the screws of slide-rest. It would be plain sulphate of copper solution? I should think the bed and headstocks, as well as all the pieces of knowledge of electro-plating to tell you how to get a job (with very little experience in filing ""), that bath. I am sorry I have not sufficient practical the slide-rest, were planed up true. Of course you the scratches, &c., out of steel knives. For plating are aware that these will all require to be brought should, however, hob them and then polish before nearer to truth than the best planer is capable of plating.-W. J. LANCASTER. doing if the lathe is to be an accurate one. Moral: You can buy cheaper than make.-A., Liverpool. [35719.]-Dynamics of a Particle.-1. S, the centre of force; B, the point of projection; B T, the direction μ force; V, the velocity. Since velocity in a circle of radius S B, force to V2 μ but force to centre =. centre = μ ... V = SB SB ... orbit is an ellipse. Again (Tait and Steele, art. 144) V = 2. (chord of curvature through S) ... cord of curv. through S=2. SB, but from Conics. SB S'B the chord of curv. through S = 2 Semi-major axis ... SB = S'B = semi-major axis, .. the point of projection, B, is the extremity of the minor axis, and the position and magnitude of the axes is now known. 2. Since the force is to be parallel to the dex . making t = 0 when no and y=0, base dt V xt μ SB =0... = SB [35726.]-Electric Light.-The cost of the lamp in itself is very small, and when properly made is nearly indestructible; the materials of the battery, too, do not come to much, but there, I am afraid, the cheapness ends. The cost of charging materials for the six-cell battery mentioned will be:s. d. Nitric acid, 2lb. This makes a total of 1 4 0 4 1 8 and as the battery will continue in action for about 20 to 30 hours, this makes d. to 1d. per hour, exclusive of zinc used. This is very high, and I have already come to the conclusion that batterypower for electric light is much too expensive and troublesome. I am engaged at present in making a dynamo-electric machine, and intend to drive it by water-power from the main, a friend of mine having invented an engine (after the model of an ordinary steam engine) that works well, and with good power. The light given by the lamp using six Bunsens will only be equal to an ordinary Now, y = a ( + sin. 6) Argand burner. It is better to join the cells up for quantity.-C. CRAWFORD-CORY. 1 + cos. 0 sin. 0 ct-say, but x = a vers. .. 1 с α d Ꮎ sin.2 0 + cos. 0 (1 + cos. ). d dy d t d t sin2 0 dt a box 8 inches long, 5 inches wide, and 8 inches 1 + cos. 0 sin. O sin. [35727.]-Drying Crystals.-You can dry acid, or if you have no air-pump, under a glass shade them under an air-pump in presence of sulphuric (air-tight) by the side of or over a little of the above-named acid.-Os. [35728.]-Clearing Forest Land.-The Duke of Sutherland, in clearing land, tore out the trees with his ploughing engines. If the trees are too large, cut them, and put a shot of dynamite in the stump, from half a pound to a pound weight, ac(1 cos. 0) or as 2 cording to size of stump, and it will tear it to pieces. A book is of no use in a case like yours; it is a practical foreman you are in need of.-M. C. J. sometimes have to fit duplex paraffine lamps in FIG [35730.]-Old Copper Coin.-There were copper farthings struck in Ireland in the reigns of James I., Charles I., and Charles II., which had on the obverse a crown, with two sceptres through it in saltiere, and on the reverse a crown over a harp; the legend on the obverse of those of James I. is JACO:MAG:BRI; on reverse, FRA:ET:HIB:REX. On the obverse the crown is nearer the bottom of the coin than the top, and on both sides the legend begins near the bottom and goes round over the top, but none of it comes under either the crown or harp. On those of Charles I. the legend on obverse is, CAROLVS:D.G:MAG:BRIT; that on reverse, same as last. On this there is a circle of dots inside the legend on both sides. The legend on obverse begins at top and goes all round the coin; on the reverse it begins to the right of the crown over the harp, and goes round under the harp, but does not go over the crown. The crown on the obverse is in the middle of the coin. On those of Charles II. the legend on obverse is CAROLVS: II: D.G. M.B.; on reverse the same as last. There is no circle of dots inside legends, which begin on both sides in the same procession as on the reverse of Charles I., and go round at bottom, could be made to give a splendid light, and heat but not at top; the crown on obverse is nearer the enough for your purposes at the same time.-WIE-top than the bottom of the coin. I think that the SENDANGER. 60 65 75 80 85 90 es ated scale, P a disc of paper with a greased central, coin which "Curious" has is one of the two first of those, that is, either of James I. or Charles I.; if the circle of dots inside legend, will show which it is. so, the possession of the letters and of the crown on the obverse, as well as the presence or absence of Either of the coins is of very little value in any case, and in the state in which the one in question must be from the description, they are entirely valueless. Those of Charles I. are more common than those of James. None of those coins are dated, but those of James I. were coined in or after 1613; of Charles I. from the first year of his reign (1625), under patent Lenox, and Sir Francis Crain; and those of Charles to Frances, Duchess Dowager of Richmond and II. from 1660 or 1661, under patent to Sir Thomas Armstrong; there were not very many of those coined, and they are often in very good preservation.-G. J. H. [35730.]-Old Copper Coin.-From the description given by "Curious," I expect this is a farthing token of James I., described in "Ruding," Vol. ii. p. 385, Sup. Part ii., Plate iii., No. 14, Obv. Legend, Iaco: D: G: Mag: Bri. Field, a crown sur of knots, and your correspondent should be sure he -PAPER MAKER. 22.32 × 100 = SO, 19-92 litres SO,. 3. The equa == 87 = 12-82 litres mounting two sceptres crossed: one with St. George's cross, for England, the other with a fleur de lis, for France. Rev. legend Fra: et: Hib: Rex. Field: the harp, for Ireland, crowned. In Vol. i. p. 369, Ruding states that, in 1613, power was given to John Lord Harrington, Baron of patent was dated on the 10th of April, 1613, for the Exeter, to make farthing tokens of copper. The often found in paper pulp. The pulp may be quite / Cl equation for action of Cl on KI is:-2KI + CII term of three years. They were to be issued within the realms of England and Ireland, and the dominion of Wales. From the name of the patentee, these tokens were called Harrington's. Thus, in Drunken Barnaby's Journal, Part iii., p. 82: "Veni Harrington, bonum omen! "Thence to Harrington be it spoken! To a beggar that did crave it, And as cheerfully receive it; For 'twas the utmost of my fortune." 79, Carlton-hill, N.W. [35731.]-Water Power.-It is not necessary to follow the stream to find the fall. It can easily be done by most land surveyors who have the instruments for such purposes. It would cost more to get these than pay a surveyor ten fees for doing the work.-M. Č. J. [35731.]-Water Power.-Select a convenient place, and make a temporary dam of clay and other focal materials, leaving a central opening, into which fix a stanch or frame of plank with a rectangular notch in its centre, of such dimensions as will include all the water, likely to pass at the time of gauging the stream (say, 4ft. wide by 1ft. deep), set the sill of the notch perfectly level, and the sides vertical. At about 5 or 6ft. back drive in a stake 4in. square, having half the thickness from its head sawn away for about 15in. in length, and set the base of the notch so made at exactly the same level as the sill. When the water of the stream has risen, and flows steadily over the sill, apply the foot rule to the base on the stake, and measure carefully the depth of water there to within 1-8ths, 1-10ths, or 1-12ths of inches, which will determine the head of water urging the discharge of the stream over the sill. Let D be the depth so measured in inches; then, for every foot breadth of the sill we have 4.75 VD cubic feet per minute passing over, or 30 D3 wil represent the number of gallons per minute very nearly. Thus; if the depth gauged be 9in. above the sill; 30 93 = 30 x 27 = 810 gallons per minute for each foot of sill. If the sill be 4ft., then 810 x 43,240 gallons per minute, would be the discharge. The power is now easily obtained, if the fall on to the wheel be given. References to sketch: -A, dam; B, stanch; C, sill; E, pond; F, bank; [35733.]-Knots in Paper.-There are several causes of this unwelcome phenomenon of papermaking, but in ten cases out of a dozen its origin may be traced to the over-filling of the pulp chest, which prevents the machineman adding enough water to open out the mass of the pulp. Any engineer knows also that pulp let off long has more tendency to knot than short pulp. The addition of abnormal quantities of loading or weighting materials, such as china clay or terra alba, but especially the latter, predispose the pulp strongly to manifest this evil. The best cure for knotting, to whichever cause it is due, is a large roomy chest in which the pulp can be well diluted before stopping for the night or for the week-end. If old papers are used in the manufacture, and have not been properly beaten out, they will cause exactly the appearance for = = 2KCI + I2. By proportion, 22-32: 254:: 12·62 :z As а 87 x 100 = -Chemical Examinations. - Read "Chemistry.-E. K. DIXON. supplementary volume, read Miller's [35733.]-Knots in Paper.-I, like "Papermaker's Engineer," have been very much troubled 50: x = 12-82 litres Cl 22:32 × 50 occasionally for years with "knots or lumps clear from knots when leaving the beatingengines; there is not the least doubt that they are made in the chests after the pulp has been emptied, and formed into lumps by the action of the agitators going too slow or too fast. I have spent some time lately with this annoying question, and tried nearly every means, but found that by driving my agitators at eight revolutions per minute is a MnO,.-T. Evans. perfect cure for the stuff lumps. Paper-maker's [35741.] Engineer" must be very careful and keep his stuff Fowne's Chemistry," the larger book of which thin in his chests, and always empty wth a regular embraces both inorganic and organic chemistry. supply of water, and not empty until he is sure that he has got room in his chests for that supply. Many engineers empty when they have not room any water at all, and the result is the paper is [35739.]-Battery.-You do not say in what full of uncleared stuff, and consequently useless for manner you tested the absence of the current. the market; but I am confident that if "Paper-you expected to "feel" it you are, of course, very maker" will look to his agitators, his "knots" and much mistaken. The best proof of the current is a his troubles will soon disappear.-B. C. P. M. small spark when contact is broken. This, I think, you will find: the battery does not require any [35736.]-Oatmeal.-"Apollo" need not harp special insulation, and will do very well on an on the process of "cutting and grinding oatmeal. ordinary bench or table. I always make my The very best may be purchased at, at least, one of batteries with copper cells, as it saves solution, and the great Co-operative Stores, cheaper than flour.- they are more portable. Put a piece of guttaSOLANUM. percha at the bottom of the copper cell, or make a cap of that material to fit the bottom of the porous cell. The top edge of the latter may be coated, inside and out, for about fin. or in. with sealingand warm when applying the varnish. The varnish wax varnish (black). The cell should be dry can be made by dissolving the wax in a little methylated spirit. About 2 ounces of acid to a quart of water will make one solution; the other a saturated solution of the cupric sulphate, with a little sulphuric [35739.]-Battery.-Your battery is a very good form of Daniell's. For constant action, put a solution of water, 20 parts, and sulphuric acid one part into the porous cell with the zinc, and a saturated solution of sulphate of copper (blue vitriol) into the outer or copper cell.-WIESENDANGER. [35739.]-Battery.-You have a Daniell's cell, apparently under made, and all that you require to work it is to first well clean all connections, then put into copper vessel a saturated solution of sulphate of copper, and in the porous jar sulphuric acid 1 part, water 10 parts; from this you will get a good current. To test simply for current, put the two ends of wires from cell to the tongue, one above and the other below. You do not require any jar to Daniell's cell.-W. J. LANCASTER. [35736.]-Oatmeal Manufacture. The oats are first evenly spread on a tiled kiln heated by a very strong fire of malting coal, and turned every few hours for 24 hours. One man takes charge of one fire and about 25 barrels. When dried it is thrown out on a cooling loft, and allowed to stand (in order to cool) for 24 hours. In the process of drying it evaporates about 2st. damp or wet from each barrel. It is now fit for the mill. It is first taken by elevators from a hopper, and thrown in on a screen fitted thus: The riddle on which the oats fall first allows the oats to pass, but nothing larger, whilst the under sieve passes out everything smaller than the oats, including small corn and a scaly substance known as screenings." It loses here about st. each barrel; it then passes to the first pair of shelling stones; these break the seeds which inclose the grain, and nip each end of the grain separating a downy substance from it called dust." Now seeds, grains, and dust, are together, and to separate the dust from the other two it passes into a cylinder covered with wire-web. This revolving rapidly dashes all the stuff inside against the perforated web, and the dust, to the amount of about 1st. to each barrel, falls through. It then passes before a fan, and from the seeds about 2st. to each barrel is blown off. Lest any grains should escape being broken, or any dust should remain, or any seeds should escape the fan, it is again shelled by another pair of stones, goes through another cylinder and before another fan, losing about st. of stuff called "pig meal," a dusty, dead oat flour. It is now ground by another pair of stones, or rather cut for sifting and dressing; all the flour broken from it by the last stone is taken off by a sieve; here the smaller grains are taken away, and well hammered thick sheet brass cut to shape, and sieve, then it goes again before a fan and on to a [35740.]-Making Concertina Reeds. -Use nothing but the large grains allowed to pass. Again file to the required thickness. it is sifted, and any grain that might have escaped little behind; if too flat thin the point.-W. H. H. If too sharp thin a unbroken rejected, to be again ground. In the last process it loses about 1st. stone of dead, fine [35741.]-Chemical Examinations.-Miller's white flour. The meal thus turned out is extra.Inorganic Chemistry," Messrs. Longman; "Qualoatmeal, about 7st. is thus taken from the barrel of 14st. In dressing wheat it yields about 19st. meal to the barrel of 20st. Now, how wheat could be double as dear as oats I do not know; and how oatmeal could be double as dear as wheat meal seems equally mysterious. However, if you think it worth the trouble, advertise your address, and I will send you the "Dublin Corn Market Report"; or get one of the Irish daily papers, Wednesday's or Saturday's, and see prices in Dublin.-PHOENICIAN. [35737.]-Lathe Bearings.-One would think that the most noted makers" ought to be about the best judges on this question. My little experience is in favour of the cone, as being steadier than the split; but I don't approve of two of the same hardness rubbing together. Experience teaches that a hard and a soft work best in this as well as in many other cases.-A., Liverpool. [35738.]-Chemical. This and the following = 174 × 100 [35740.]-Making Concertina Reeds. - It doesn't pay to make these one's self when the usual charge for putting one in is only three or four pence. They are purchased, say by the gross, of all sizes, and, picking out the nearest size to what you want, you fit it and rivet in, and then tune it. You might just as well attempt to make your own steel pens or pins.-A., Liverpool. itative and Quantitative Analysis," by Fresenius, Messrs. Churchill; and a reference to Brand's "Dictionary of Science," Messrs. Longman; "The English Cyclopædia of Arts and Sciences," Messrs. Bradbury and Evans; "Ure's Dictionary of Arts, &c.," Messrs. Longman; The Journal of the Chemical Society, and the Transactions or Proceedings of the Royal Society, the Cavendish Society, the Royal Institution, "Die Jahresbericht der Chemie," 20 vols.; try consult Armstrong, Frankland, Miller, Watts Wurtz'&Watts' Dictionaries, &c. For organic chemisand Wurtz's Dictionaries, and a work on the eve of publication, by Messrs. Blackie & Son, adapted to suit the requirements of elementary, advanced, and honours students, containing the answers to the papers set during the last ten years, on the three stages of organic chemistry, in an appendix with other useful matter.-HUGH CLEMENTS. [35741.]-Chemical Examinations. — Read Messrs. Miller's "Inorganic Chemistry" and Frankland's "Notes," then work as many of the analyses as you can out of Fresenius" "Qualitative." This latter part of the work must be done well, or you will be plucked. You should also get access to the Journal of the Chemical Society, and read up all specially new processes to within three months of date of examination. Get up the "Graphic Formula of the Silicates," &c. Fororganic, read Miller's "Organic," and continue through the organic in Fresenius. Read also Frankland's "Notes," and you will find the Journal of the Chemical Society useful. Don't attempt reading every book you can get hold of, but go through a standard work systematically, and make notes always. Always make notes-they not only help you at the time of making them, by allowing you to work longer, but they are of enormous value to you long after the examinations are over. I have passed some 60 examinations, and above all the value of books and certificates are the notes I made, and which have now become stored in my brain. I would advise everyone in all their making to make notes (not bank notes).-W.J. LANCASTER. [35743.]-Fastening Hand Rests. -I send the following rough sketch :-A is a cast-iron washer has not yet come under my notice-at any rate [35753.]-Transparent Colour.-"E. B. F." W. T. O. [35756.]-Paper Sensitive to Electric Cur- well with three or four cells.-W. J. LANCASTER. [35761.]-Intensity Coil.-To Mr. LANCAS- will suffer; the oval form indicates want of adjustment. Don't polish in straight strokes, but in the smallest circles you can, taking the polisher round a circle midway between centre and margin from graduals going up to centre, thus making a succession of small spirals from margin to centre.-W. J. LANCASTER. [35780.] Varnishing Chromos for the Lantern.-Nothing is better and freer from bits, &c., than a thin layer of Canada balsam. Get the ordinary Canada balsam, thin it out with turpentine, drain off excess and allow to dry only a little, then warm both the balsam and glass, pour on evenly, mount with the second glass.-W. J. LANCASTER. [35781.]-Mounting Objects in Canada Balsam.-Assuming that the object to be mounted has been properly prepared, then place it upon the glass slide in the position you want it to have, slightly warm the glass and carefully put one spot of Canada balsam over the object; warm the glass the cover glass on in a wedge form, gently pushing a little more to make the balsam spread, then put the balsam in front of it, but not sufficiently quick to displace the object, when the glass is down flat a small spring should be put on it to keep it down; then place it in a warm chest for a few days when it may be taken out, cleaned, named, and put in cabinet.-W. J. LANCASTER. [35783.]-Telephone.-Too strong a battery will spoil the effect of the best microphone: probably herein is your error; have less battery power. Again, the carbon rod must be slanting for speech transmission, or you will get a considerable jar. with it. A thin resonant box placed over the Bichromate will not do alone; you must use acid microphone greatly improves it; as a transmitter of speech and music the box need not be perforated. -W. J. LANCASTER. [35785.]-Lens.-The maker of your lens is not 4 pint bichromate cells. I would have more secondary for under the lathe bed; B, eccentric and shaft; wire, say 8oz.; this would give you a good ĝin. a maker of any note; but at the same time the lens C, square on end of eccentric shaft for wrench or screw-key; D, the socket and eye for the eccentric spark, and would go up to a thin in. For cheap- would not work quick in such dull weather as we ness you cannot beat the bichromate. The materials have had lately. It may work fairly well, and for to work in the hole in the top of the socket is tapped for 4 pint cells will only cost you 3s. 6d., so that the price, I have no doubt it will be good.-W. J. out for the bolt E to screw in. F is supposed to be these would be much cheaper than the Bunsen's. LANCASTER. a section of rest showing the slot where the head of bolt E fits. SS are two bearings standing up from the washer for the eccentric shaft. There is another way that this can be worked, but you would have to get a new rest, and I like this plan the best.-A. HAWORTH, Bacup. -W. J. LANCASTER. [35767.]-Electric Light.-You cannot produce a light equal to only four gas-burners except by incandescence, as to which I can scarcely give exact figures as yet. The nitric acid cells are the only ones of use for the purpose, and require 30 or more to be useful. Carbon rods one-eighth of an inch thick would be used for small lights.-SIGMA. [35744.] Coal for Steam Engines. "Graique's" question depends so much upon firebox surface, Hlues, and drawing of blast, and whether the question of smoke is a consideration or [35767.]-Electric Light.-To Mr. LANCASnot. Considering, from his silence on the subject, TER.-1. You cannot get a light from carbon points the latter is of no consequence, I can strongly equal to four gas-burners, the smallest light you can recommend, in the absence of Greenland coal in get of any value will equal any ten gas lights, this the market, Walker's Morley Main for efficiency you can get from 16 pint Bunsen cell. 2. The curand Wright's Newcastle Wallsend deep for cheap-rent would be continuous for 25 to 30 hours after ness.-0.0. being charged. 3. The carbon should be 5in. long and in. square, for pint cells.-W. J. LANCASTER. [35770.]-Residue from Oxygen Mixture.The residue consists of KCl + MNO,; the KCI dissolves in water, and the manganese will not, so wash the residue, pour off the liquid and let it dry. -C. CRAWFORD CORY. [35746.]-Small Lathe. I would willingly oblige "B. K.," but I have no experience of so small a lathe, and look upon anything under, say, 6in. centres, as only a toy.-A., Liverpool. [35748.]-Pure Gold.-One of "Bob Lowe's" economies was replacing the pure copper used in the Mint for alloying the gold coinage by some "cheap and nasty" alloy or mixed metal, which had the effect of making the sovereigns in which it was used brittle, and also prevented them from ringing, which caused a good deal of annoyance to many, as though they were of course perfectly good and a legal tender, it was impossible to get the wellknown ring of the genuine coin out of them, either on a counter or even a flag, as a chimney-piece or hearth-stone, and they were so brittle that they were sometimes broken in the attempt. Now I think that it is exceedingly probable that "Dunstan" had the ill-luck to have used one of those "Bobby's," and that his manipulation being adopted on the supposition that the gold was alloyed with pure copper, his failure, like many others of far graver import, was caused by Bob Lowe's penny-wise economy. The sovereigns in question have the George and Dragon reverse; I am not quite sure of the date, but I think it is 1872.-G. J. Ĥ. [35749.]-Electric Light for Launch.-An electric light could be fitted into your steam-launch. The first outlay would be £70 for the machine, and £5 to £10 besides for lamp and fitting. The actual cost per hour afterwards, power and attendance to engine, &c., not counted, would be 3d. to 6d. per hour.-WEISENDANGER. [35749.]-Electric Light for Launch.-You cannot at present have a machine to be driven with less than a 2 horse-power, and this you could not, I imagine, spare well. To get a steady light, at least 4-horse is used up, and then the light is not so successful when the machine is shaking about. For your purposes batteries would be the best, and could go in the bottom of the launch or in any stowaway part, and for them the shake would be of value in increasing the light.-W. J. LANCASTER. [35752.]-Celeroscope.-I am sorry this slide [35786.]-Great Eastern s.s.-Her tonnage is 28,000 tons, and her engines of from 10,000 to 12,000 horse-power. I think she is at present at Milford Haven.-W. T. O. [35802.]-Defective Leclanche.-It is simply worked out, and requires a fresh charge of manganese. It is absurd to buy secondhand Leclanchés except for this purpose.-SIGMA. [35802.]-Defective Leclanche.-The carbon binding screw is probably corroded over. Clean all connections well, swill the porous jar, or let it stand in warm water for an hour, clean the zinc, then put together again, and recharge; the cell will then go well. Of course, a Leclanché will not continue working for any lengthened period of time. It is well to use it only a few minutes at a time, and then let it rest for an equal period of time.W. J. LANCASTER. [35806.]-Bicycle Spring Step.-There is a spring bicycle step. It is used to the Pegasus bicycle, I believe. I should not however recommend it, as it is apt to get weak and then is no good as a spring step or an ordinary one, and a new one is necessary. A saw step is a good one; I use it on my bicycle.-T. M. S' [35770.]-Residue from Oxycalcium Light. -All you need do is to wash the waste out of the retort, let it run into a tub, well stir up, and break up the lumps, then let the manganese settle, pour off the water and dry the black powder, pulverise [35836.]-Test for Arsenic in Wall Papers. and use over again; but considering that manganese-A rough and ready test for the above is to scrape is worth 2d. per lb. it is not worth the trouble.-W. a little of the colouring matter off the paper, and J. LANCASTER. [35775.]-Bicycle.-I should advise you "C.R." to get a Challenge No. 2, by Singer, Coventry, extra thick tires, for your son. It will not injure him if he does not lean over the handles much. I should also advise "C.R." to purchase a second-hand machine, as he will soon want a higher one. Tell me his size and then I can tell size of bicycle.-T. M. S. heat it on the blade of a knife, when, if arsenic be present, its peculiar alliaceous odour will become manifest. If a more delicate test be required, there are Marsh's, Reinsch's, Lassaing's, and a host of others to be found in almost any chemical work.-R. PEARSON. [35837.]-Cory's Electric Lamp.-1. The size of the brass tubes is quite immaterial; you must adapt them to the size of the carbons you intend to [35776.]-Reflecting Telescope. To Mr. use; half-inch will do, and it should slide freely LANCASTER.-In the first place, I should recom- into tube a trifle larger. The length of tube for mend that the polisher be smaller then the speculum: positive holder, about 18in.; for negative, Sin. A this will enable you to work much better than by wooden or other kind of lever, 6in. long for the size having polisher just the size of speculum.. The of lamp described in my letter, will do as long as main point in connection with flat is that it shall be the insulation is preserved between the carbons. I in the optical axis of speculum; this of course has have made the lever and connecting rods brass, one nothing to do with the tube, but it is well for many end of the latter screwing into ivory ends, which reasons to have the flat directly in the centre of effectually insulates them. The fulcrum of the tube, and to adjust the speculum to the position lever is supported by a wooden upright by a screw of flat; by this method you get a clear aperture let into the lever and screwed into the upright. I right through body, whereas by moving flat do not understand what you mean by "stop at E, to centre of speculum, you may lose a portion fig. 5." It is a pin on which the connecting rod is of field beside bringing reflections from side of tube free to turn. The bobbin consists of four coils of on to speculum; this must be avoided. The tube (No. 14 or 16 B. W. G.) covered wire, five inches should be about one-seventh larger diameter than high, one end of the wire of which is fastened to a speculum. A few rings same size clear as diameter binding screw (marked-), the other to the negaof speculum should be put in various parts of tube tive holder; the positive holder is connected to to prevent any stray reflection. The inside of tube another binding screw (marked +). The end caris better blackened with a good dead black. The bon of the battery is connected to the binding screw markings you can see are due to the fact that the+, and the zinc to the one. I should mention polishing has not been well done; this is the part of that to obtain a good light, 30 cells must be used. the work which must be well done or the definition 2. The gas jar is placed in water after the phos phorus is lighted, as shown in the illustration.-C. CRAWFORD CORY. [35838.]-Conversion of Bunsen into Bichromate Batteries.-Your plan would prove a very wasteful one. The negative surface should be as large as the positive one, when possible; whereas you propose to have a carbon of 10 inches superficial area opposed to a zinc cylinder of 45 inches internal surface in addition to its outer surface, which would be consumed to no purpose. If you do not wish to go to the expense of larger carbons you had better make your zinc cylinders much smaller (14in. diameter at most); you will thus reduce the expense without losing efficiency. Bear in mind that no porous cells are required, but the carbons must not be allowed to touch the zincs. The solution for charging is made as follows:Dissolve 24 ounces of bichromate of potash in one pint of water, and when cool add one fluid ounce of sulphuric acid.-R. PEARSON. [35839.]-Amalgamating Zinc.-Pickle the zines in dilute acid for a short time, and then place them in mercury, rubbing the troublesome parts with a hard brush.-C. CRAWFORD CORY. [35839.]-Amalgamating Cast Zinc Plates. This has just been explained by Mr. Wiesendanger, page 419, in "Notes on Electric Batteries, III."-W. T. 0. [35844.]-Legal.-The best course for "A." and "B." to pursue is, either to leave the deeds with the solicitor who now holds them, or to place them in a bank or some other place of deposit, subject to the condition that they shall be produced to each party at his own request, but shall not be handed over except upon the joint order of both parties. If either "A." or "B." had possession of the deeds he would be entitled to retain them, subject to the right of the other party to demand their production for his examination at any time.-SPERANS, Lincoln's Inn. QUERIES. [35852.]-Coke Burning-I have been engaged for many years in burning coke for locomotive use. Lately, I have been requested to supply a different kind, suitable for smithy purposes, which, it appears, should be much Will some contributor point out the simplest known means softer, partaking more of the nature of common gas coke. of accomplishing this? Can it be done with Tanfield Moor alone, or must other kinds of coal be added? It has been suggested I might succeed with small charges of coal kept continually broken up during the brief period of combustion. Is there any hope of success in this plan? I about 3 tons (of coke) capacity, the air supply being may mention my ovens are of the old-fashioned type of regulated merely by a damper. Does any literature in a cheap form exist upon the subject of coke burning? I have Tomlinson's Encyclopedia, and Wye Williams's work; but their costliness and the absence of any library in this locality prevent me consulting Ure or Muspratt.W. B. W. [35853.]-Photo Lenses. -Willany optician enlighten me on the following subject:-If a c-d-v combination of 4in. focus takes a standing figure of 24in. at 15 or 16 feet distance, would a combination of 9in. focus take a cabinet 5in. standing figure at the same distance! If not, what would be the best length of focus to obtain that result, as more than 16ft. of studio cannot be made available? Or if lesser length of figure be more suitable for standing cabinet, say 4in., what length of focus would give that? -CYMRO BACH. [35854.]-Transit Instrument.-Being about to buy a transit instrument, I am desirous of having a means of recording, by electricity, the times of transit on each wire, to tenths of seconds, and I shall feel obliged if any of your readers would kindly either give me instructions as to the best way of so doing, or could tell me if such an instrument is made, and, if so, where I can procure one, and about the price. My observatory is large enough to place another clock in it, I have a good sidereal clock that beats seconds.-Wanderer. Can all the but both differ from a mounted object I have, which is the best works to read on diseases of the eye and defective vision-MYOPS. [35864.-Raising Powers in Arithmetic.-2 raised to the millionth power contains 301 031 digits. Will any of" ours" say how this can be ascertained without going through the actual tedious process of figures, whether even logarithms will help one out of it? Instead of taking in the ordinary way, if we square every product, thus, 24 = 4, 4% = 16, 162 = 256, and so on, with 256 and the remaining up to a million products, what number of digits will the last contain !-TYRO. [35865.-G. N. R. Coupled Engines.-What are the principal dimensions of No. 18 driving and leading wheels coupled, built at Doncaster in 1867? Are the driving-wheels 5ft. 6in., and cylinders, 17 x 24!-C. R. M. [35866.]-Colours of Railway Engines.- What colours do the principal English railways paint their engines? I know that some of the engines used to be painted green on the G. W., G. N., G. E., L. and N. W., &c., brown on the L. and S. W., blue on the Caledonian, yellow (I think) on the L. B. and S. C. Are these the colours of all the engines on those lines now?—C. R. M. [35867.]-Star Magnitudes. -Will "F. R. A. S." or some other astronomical contributor kindly publish some of the methods of accurately determining starmagnitudes! I use a 4in. refractor by Cooke, but have not got an astrometer. Such work, I believe, can be very simply done without special instruments.-RIGEL. [35868.]-Striking Out Letters in Sheet Metal. -Can any correspondent inform me of a simple and cheap plan for striking out different shapes, such as letters in sheet metal of 32 B. W. G., so as to leave a burr round stamps and dies will not do, as they are too expensive for the edges of about 1-32 of an inch in height? Ordinary small quantities.-J. M. [35850.]-Legal.-I do not think that the receipt of rent, with knowledge of the breach of the covenant in question, would waive the right of forfeiture. I consider that the not building would be a continuing breach, and might be taken advantage of at any time. I do not know that the exact point has been decided, but it was argued in a case of Herring v. Bennett before the Court of Common Pleas in 1857 (3 C.B. N.S., 370). In the view of that of the current is the same in both, and attracted if the about it, as I have been in search of such a thing for years. case taken before the judges who heard it, the point was immaterial, but two of them expressed opinions in favour of the view I have adopted.SPERANS, Lincoln's Inn. [35147.]-Finishing Ends of Rails (U.Q.)As no one has answered this, I will contribute what little I know on the subject. A few years ago I had an opportunity of witnessing the process of rail rolling, at a large ironworks in South Wales. The method adopted was to saw off the ends while red hot, from the rolls, with circular saws, and finish off the burr with coarse files. I noticed that the saws worked in pairs (one for each end of rail), but this may have been a matter of convenience. I have seen some light contractors' rails, of the ordinary "bridge" pattern, with the dovetail groove as per sketch. A relative of mine, who has been in the roll-turning business for years, told me the rails are first rolled with the sides of groove parallel, and outsides inclined to each other; but in passing through the rolls the last time, the sides of the groove in the roll which form the outsides of the rail are parallel, and press the sides inwards, and forms the dovetail, the collar on the corresponding roll being of the width of narrowest part of dovetail.-A. D. P. [35855.]-Omnibus Checks.-As it was in the ENGLISH MECHANIC I saw that the London General Omnibus Co. had offered £1,000 for an invention for checking the amount taken by the conductor, I take the liberty of asking if it has yet. been awarded, and if they still offer that sum, or any other for the said invention!-E. S. C. Two currents, which are parallel, and flow in the same diree[35856.]-Electro-Magnetism. - To "SIGMA."tion, attract one another; two currents, which are parallel but flow in contrary directions, repel one another (Ampère's law, "Cassell's Popular Educator.") If the wires of two circuits are laid alongside of each other, and are free to move, they are mutually repelled if the direction currents are contrary. ("Chambers's Electricity," page 81.) Which of these statements is correct! Suppose we clasp a tree in our arms and that a positive current circulates from the right hand across the breast to the left hand, the top of the tree would, in this case, represent the North Pole, and the root the south. (Chambers's Electricity," page 84.) Let the observer imagine himself placed along any portion of the wire with his face turned towards the centre of the helix, so that the current may enter by his feet and leave by his head, the North Pole Educator.") Are not these two statements also contra-machine for a man 5ft, high, and oblige—O. S. will then always be at his left hand. ("Cassell's Popular [35871.-Bicycle.-Please give proper size of dictory -ÚPSILON. [35869.]-Waterproof, Glues.-I noticed some time ago, a company windir ap with the above designation. Is there such a thing as glue being waterproof after setting? If so, I would like very much to know something Could any of your numerous readers or correspondents give any information on the subject-M. C. J. [35870.]-Water Wheels and Turbines.-I much want to know the power of two water wheels, one 30, and the other 15 feet in diameter, both overshot, and each driven by 500 gallons per minute, and what difference in power would it be if they were breast wheels; also what size and description of turbine would be equal in power? L. GILBERT. [35572.]-Bleaching and [35857.)-Mortgage.- Will some of your legal would be much obliged if any of your correspondents Filtering Oil.-I readers kindly give me the point of law on the following: could tell me how to bleach linseed oil. A lends B a sum of money on his house, and a mortgage possible, quite free from colour. I have tried whiting, I want it, if deed is duly drawn up. Years after, A makes his will, animal charcoal, sulphuric acid, without, or at least with disposing of all his money, but omits to mention or devise very partial, success. I should also like to know the his mortgage estate, consequently it falls to C, his heir-at-readiest way to filter oil through fine animal charcoal !law. D, who has been appointed executor, requires C to A. E. R. the deed was not devised, D has no control over it. sign a deed of transfer to himself; but C contends that as Cannot C claim the title deeds of B's house until the same are redeemed --C. W. W. [36858.]--Hot-Water Apparatus for Bath. [35873.]-Night Sweats.-For some weeks past (upon awaking in the night time) I have been subject to these sweats, especially the extremities, such as the legs Should like to know what is the cause, and a remedy, if and head; do not seem to sweat so much about the chest, any; I would add that I feel pretty well in health, &c.ANXIOUS. [35874.]-Enamelling Iron.-Can any reader give me the correct name and number of specification for The patent, I believe, was taken out by a firm in Staffordenamelling iron called Willing's Patent Enamelled Iron? shire, Willing being only the agent and not the patentee of this process. Not knowing who really is the patentee or the number of patent, I cannot obtain the specification. -CERAMI. [35875.]-Heel-ball Solvent.-What is the liquid used by shoemakers to dissolve the heel-ball. I have tried of naphtha. Shall be glad if some fellow-reader will give wood naphtha without success. I am told it is some kind the names so that a druggist would know the kind wanted. [35876.]-How to Make a Holtz Machine Work.-I should feel much obliged for information respecting the making of a Holtz Horizontal Electrical Machine. I have just made one of the above, but it does not answer. There are 2 glass plates, 14 diam. 5-16 thick. each plate runs in opposite direction to the other, and the teeth in the combs are about 1-16in. off the glass plates. Would you say if that is the right distance, and which is the best method of exciting the machine? I have taken the design out of the ENGLISH MECHANIC, No. 398, Nov. 8th, 1872. I should be glad if "Sigma" would answer this.-J. E. H. [35860.]-Twaddel's Hydrometer.-Would any [35877.]-Bent Lever.-Find geometrically the centre of gravity of a bent lever of uniform thickness and density.-AKROOMENOS. (35878.]-A Cone.-From a cone, whose height is Sin. a similar cone, 4in. in height, is cut off by a plane parallel to the base; what is the distance of the centre of gravity of the frustum from the base !-AKROOMENOS. [35879.]-Projectile.-A ball is projected with a velocity of 966ft.; what must be the angle of elevation that its horizontal range may be 9,660 yards!-AXROO MENOS. [35890.] Power of Induction Coil. Would some friend assist me in the following:-My coil is 10 in. long, with core 14in. diameter, and has about 1b. of silk-covered No. 34 secondary on (the winding is not yet finished, so if more wire would be an advantage, I can add it). With two quart bichromates, single carbon plates, and putting the current through a small coil for a contact-breaker, the coil gives to in. spark. I want to know the proper battery power to use, and what length of spark I may expect from it. The coil is wound in two sections, divided by diaphragm, and there are 14 coils on one, and 15 on the other.-CHARLES ATKINS. [35581.-G. W. R. Engines.-Would any correspondent have the kindness to give a short description of the type of 4-coupled passenger engines now working the traffic on the broad gauge between Paddington and Bristol-W. at [35882]-Telescope Adjustment.-I have a large landscape telescope on garden stand, which I use for taking occasional peeps the planets, &c. but I find great difficulty in obtaining an accurate focus on account of the absence of any rackwork adjustment. Would any of our readers suggest a plan of fixing a rack and pinion to the eyetube that could be removed when required so as not to interfere with the shutting up of the telescope ?-D. B. [25883.-Pianoforte.-I have a piano which will not keep in tune a day. What is the probable reason? I do not think it arises from the pins being loose.-DANDIE DINMONT. 96884.]-Quarter Clocks -Will "Seconds' Practical Watchmaker" kindly explain the repeating action of the quarter side of an ordinary quarter clock (small). I understand very well the going and striking parts, but not the quarters.-A. H. K. MAN. [35894.1-Eyesight.-To Mr. LANCASTER.-I think I have not made this query plain enough. I do not require glasses for reading purposes, but to preserve my sight, which is gradually getting worse. The farthest point at which, without glasses, I can read the "Replies" is 134 in., and the nearest 3in.; age 24; occupation, engine fitter. Would you kindly give a few hints as to the care of the eyes.-HOPELESS. [35895.)-Electricity.-Would anyone kindly draw a plan of the single line arrangement, and explain how I can call the attention of a distant friend with bell, then speak with telephones, and, if necessary, use the microtelephone transmitter with a telephone to get the answer, the former requiring the battery, and the latter not? I know it all lies in the switch, but that is all.-EUREKA. [35896.]-America.-Will anyone kindly give information respecting towns in Virginia or Tennessee, as to rent, price of provisions, &c. Also, whether a lady, holding a first-class diploma from Stuttgart, Royal Conservatoire for piano, the Associateship of College of Organists, London, and with a perfect knowledge of French and German acquired abroad, is likely to obtain remunerative employment ?-EUREKA. [35897.]-Battery-Would “ Sigma," or any other able correspondent, kindly answer the following questions? -I have made a 4-gal. battery, with 1 zinc and 2 copper plates, acidulated with 1-15th sulphuric acid, the zine plate I well amalgamated three times, but on placing it in its own solution by itself, in two minutes it caused great effervescence as much as though the battery was in circuit; ought this to be so? Where can be the fault? On washing and wiping the plate it had a good silvery appearance, without dark spots. Is this battery suitable for plating, say a teapot! If not, what kind and size is recommended? I can plate and gild small articles with a quart Bunsen but cannot obtain the frosted appearance; why is this? Am about making an electric lamp; the weight of the carbon to regulate it, can I get a light equal to one gas burner from this, and how long would it last? The kind, No. of cells, and size required; I want it to burn two hours.-AMATEUR HARRY. that out of 511,745 males (the proportion of boys in 1,000,000 births), one half are alive after 44 years; and it would seem that this would therefore be the expectation of life for a male infant. But on p. 216, is a table, also by Dr. Farr, of expectation of Life, in which that of boys at birth is stated to be 39'91 years. If half the boys born live to 44 years, why should the expectation of life for a male infant be only 39 91 years?-J. E. A. [35909.]-Lantern Tranparencies.-Will any correspondent tell me how to photograph diagrams, engravings, &c., for magic-lantern transparencies? Is it possible to transfer engravings directly to glass for transparencies? I have tried the gum damar process without success.-FILTER. [35910.]-Analysis of Oils.-To MR. ALLEN. -Will Mr. Allen kindly inform me how to estimate mucilage in fat oils, also if an admixture of water with oils can be estimated by loss of weight upon heating in a platinum dish over the water-bath! I should be glad to know, too, the physical and chemical characteristics of a good lubricating oil, and any other hints on the subject of oil analysis, or reference to authorities, that he may be good enough to furnish me with ?-PETER OLEIN. [35911.]-Holosteric Barometers.-I shall be greatly obliged for information as to the derivation of the word "holosteric."-X. [35912.]-Holly.-I have a quantity of holly-trees from 24in. to 40in. cir. Can someone tell me if they are of any use; is anything made of this wood!-IGNORAMUS. [35913.]-Electrical Alarum.-I am about fitting an electric alarum in my servant's room, and as it would sometimes have to ring for ten minutes or more together, would a Daniell's battery be better than Leclanché, as I am told that the latter should only be used with intervals of rest.-G. A. P. [35914.]-Chemical Food.-Would some reader tell me the composition of Parrish's Chemical Food," as I have been told that it is better than cod-liver oil?-Cox SUMPTION. [35915.1-Dyeing Sheepskin Mats.-Would any of our correspondents inform me how to dye a glossy jet black on sheepskin mats! I have tried several methods, but cannot get one to my liking?-NOVICE. [35885.]-Winding Bobbins.-Would any reader inform me a method of winding electro-magnet bobbins for electric bells, to do it neatly and with speed! A sketch of any kind of machine for same that could be attached to a lathe, or separate, with instructions as to its construction would be greatly esteemed.-A SCOTCH[35898.]-Mechanical Conversion of Motion. (35886.]-Canoeing-Is it possible to get from-Can any subscriber give a sketch or diagram showing Kingston-on-Thames to Barling, near Rochford, Essex, how the mechanism is arranged with the engines in the in a canoe? Can anyone describe the route, or tell me if I Houses of Parliament, for the purpose of guiding the can get a map showing it?-A. P. B., piston rod in a straight path in accordance with article on [35916.]-Newtonian Reflector.-To Mr. LANthe above subject, at page 269, Conversion of Motion or for CASTER. -Wishing to make a small Newtonian reflector, producing rectilinear motion from circular? In one of 4in. diameter, 3ft. focus, would you kindly inform me if a the numbers of the ENGLISH MECHANIC, on the same sub-plane concave mirror would do, and if not, how am I to ject, viz., Mechanical Conversion of Motion, the following get the parabolic curve? Also, what should be the sketch was given with the following remarks placed with diameter of the flat, and also the diameter of the eyepiece tube? And what would be the highest power I could use to such a telescope? Any other information with respect to the above would greatly oblige-J. E. H. age. [35887.] - Oboe Playing.-I am about thirty years of I have had no practice with wind instruments at all, but I am fairly acquainted with the technicalities of music, have a knowledge of thorough bass, play the piano, &c. Having a desire to take part in an orchestra, I should like to know whether the peculiarities of the oboe are difficult or easy to master. The answer, I know, is somewhat difficult to make, as the element of the pupil's aptitude is to be taken into account; but perhaps your obliging correspondents will tell me. Supposing a scale exhibiting the degree of executive difficulty of orchestral instruments, what place would he assign in it to the oboe; secondly, whether an individual of ordinary ability at my age would be able to attain such proficiency as to justify him in taking up the practice of the oboe; and lastly, if so, what course of study he would recommend?-UPSILON. [35888.]-Photographic-How can I obtain dense plates by the gelatino-bromide process? I am using the formula that "Finem Respice' gave in July, 1878, in "ours," except, instead of precipitation with methylated alcohol, I dyalised the emulsion to get rid of the nitrate salt. I am developing with the liquid ammonia alkaline development; but even after intensification with silver, I cannot obtain the plates at all dense; the sky is of a light transparent brown. I tried the formula for the ferrous oxalate development that "Finem Respice" gave me, in a dipping bath, but could not even obtain the slightest traces of a picture. Docs drying the plates in methylated alcohol, after the emulsion is set, cause frilling E. C. R. it, viz., A sketch of a mechanical arrangement of levers [35917.]-Compound Engine.-Would any kind reader give sketch and describe the valves of a compound engine, such as drive some of the merchant screw steamboats; Also the proportion of one cylinder to the other, also a description of the surface condenser generally used? Such information would oblige-J. E. H. [35918.]-Canoe.-I am making a canoe on the principle given by our friend Mr. E. Jackson in No. 705, Sept. 27, 1878, and I think of fitting it up with a screw propeller; will some of our kind correspondents say what size pitch of blades and number of revolutions would be best per minute, also what speed I may expect to get? It is to be worked by hand, and for a canal or small river.-AN OLD READER. Examinations. Will somebody please tell me what mathematics are necessary and double first engineer, in the merchant service? Also to enable one to pass the examinations for second, first, What books to get which would afford me additional information on above subject.-ENGINEER'S APPRENTICE. [35919.] Engineer's [35899.1-Cost of Cloth.-I am engaged in the jute YOUNG OVERSEER. (35900.]-Gas Coke for Steam Boilers-Would any reader inform me if gas coke will injure my boiler, as my engineer tells me it will? I have always used coal; however, as gas coke is cheap it would effect a great saving. I use a cylindrical tubular boiler, with external fire returning through 4 inch tubes, and thence back along the sides to chimney.-STOKER. [35901.1-Bessemer's Gold Paint.-I shall be greatly obliged if anyone will tell me how Bessemer's gold paint should be prepared for writing with ?-X. [35889.]-Photograph.-To "FINEM RESPICE.". [35891.] Creosote. -Would any correspondent kindly state, briefly, the different processes employed in the manufacture of this substance ?-E. K. DIXON, [35892.-Resistance. In railway motion, what is the average resistance per ton of the weight of the train ? -E. K. DIXON. [35893.]-Highway and Paving Rates.-Can a local board compel payment of highway and paving rates, from tenants whose houses abut on roads which the board have declined as yet to take to? The road in front of my house is simply grass ground in the summer, and, at present, an impassable quagmire; on the opposite side is a narrow path maintained by the local board.-KAPPA. [35904.-Refining Silver-Will anyone inform me of any simple method for refining a small quantity of silver which contains silver and, I suspect, pewter solders? J. S. A. [35905.]-Cost of Working the Bunsen Cell.Can any reader inform me what is the cost per hour for producing the electric light from a 20-cell Bunsen battery, also what weight of zinc is consumed per hour, and also, how many candles will the light from 20 cells give?— BEGINNER. [35906.]-Screw Centre. -How may I set about making a screw centre for my lathe, such as is figured on P. 536, No. 724, in fig. 6?-A. D. P. [35907.]-Boiler Composition.-There is a solid composition sold by Andrew and Co., for the prevention of incrustation in steam boilers. Can anyone tell me of what it is composed? And also, if its action will be improved by the addition of soda?-A. D. P. [35908.]-Expectation of Life.-Will any statistician enlighten me as to the mode of calculating expectation of life! From a table by Dr. Farr, F.R.S., published in Whitaker's Almanac, p. 215, it appears [35020.]-Volumetric Analysis.-Is there any good reducing agent used as a standard solution in volumetric analysis, which can be readily prepared in a pure state and does not oxidise on exposure to air? Can ferrous sulphate be ppted, as FeSo,+7HO, by addition of alcohol to its aqueous solution? And can it be depended upon as being quite pure after being dried in air at the standardising solutions for oxidation and reduction proordinary temperature? What is the best substance for cesses? Can cuprous chloride be dried in air without oxidation ?-P. T. O. [35921.]-Lenses.-To Mr. LANCASTER.--Would this gentlemen, at his convenience, kindly favour a novice with information on the following points :--1. Which description of lens, by Ross, Dallmeyer, or Grubb, would he, as an optician, recommend (irrespective of makers' interests), as the very best lens for copying purposes, say charts, maps, and plans? 2. What may be considered the utmost limit permissible in the difference of size of any object in the two pictures constituting a stereoscopic view? 3. Is Grubb's method of determining the focus of for a difference of a tenth of an inch in the foci to exist in a lens scientifically accurate? 4. Is it an unusual thing a pair of lenses of so-called" identical foci?" In connec tion with, and in explanation of, the last three queries, I wish to remark that there has recently come into my possession a pair of "stereographic" lenses, stated to be of identical foci." These lenses I have not yet had the opportunity of testing photographically, but by Grubb's method I find their foci to be 52 and 5 14 inches respectively, thus indicating a difference in their foci of '06 of arithmetic be correct, a displacement or difference in the an inch, as a consequence of which there may arise, if my size of the pictures of the same object on a stereo-plate of 0:33 of an inch, and a difference in the exposures required to produce the same intensity in the two parts of the negative of, perhaps, 2 per cent. The focal difference (06) can, I believe, be eliminated by unscrewing the back cell of the shorter focus combination; the amount necessary I have computed to be 12 of an inch nearly. I have ference of foci would show itself in the difference of an idea that the more manifest consequence of this difintensity in the two halves of the negative, rather than in any inconvenience arising from such a minute difference (033) in the size of the pictures, the eyes possessing such a marvellous power of accommodation as respects superposition. An opinion on the foregoing points from a specialist such as Mr. Lancaster would, no doubt, prove as acceptable to others as to-CUIDICH N'RнI. |