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pipe condenser, whereby it might be thinner and lighter; but I fear this would be too subject to accidents. Let us suppose,

however, that it could be reduced to 300 lbs. weight, which, with the water, would make 660 lbs.

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"The fly, 3 feet diameter, containing the power of one stroke

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"And if the organ-pipe condenser is not found sufficient to
condense the steam, water must be carried, say for two
hours

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2900

“And then, if the machinery cannot be made lighter than has been stated, the power will, I am afraid, prove insufficient, and a further augmentation of the boiler, &c., must take place. If there were no friction, the power of 80 lbs. would be sufficient to draw the carriage up an ascent of 1 in 36 on a hard smooth plain; but there would be some friction, and roads are both unequal and generally soft, which latter gives a continued resistance of the same nature as going up a steep ascent. There is another consideration, which is, that the carriage, being loaded with the weight of the engine, would require stronger wheels than usual for these machines, which would still increase its weight.

“I have taken it for granted all along that 80 lbs. is a sufficient force to move a common post-chaise, loaded as I have mentioned, (but without the engine); which, however, I am by no means certain of. I rather apprehend it will require that power on a quite plain road, and more upon acclivities; for four men would be able to exert that force, and yet could not move the loaded post-chaise at the above rate; and I observe that horses labour as much in a cart as they do in a horse-mill, yet they

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LOCOMOTIVE STEAM-CARRIAGES.

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exert near 200 lbs. each in the latter, at the rate of 11⁄2 miles per hour.

"31 August, 1784.

"The whole matter seems to turn on an answer to the question, whether 80 lbs. be a sufficient power to move a post-chaise on a tolerably good and level road at the rate of 4 miles in an hour. 2dly. Whether any less surface than 8 feet exposed to the fire be sufficient to evaporate a cubic foot of water per hour, without much waste of fuel; which question seems partly answered by the boiler of our corn-mill, which has only about 5 feet surface exposed to the fire for each foot it evaporates per hour; and it evaporates about 8 cubic feet per cwt., so that by submitting to a waste of coals, a smaller boiler would do. 3rdly. Whether it will require steam of more than 11⁄2 times atmospheric density to cause the engine to exert a power to 6 lbs. on the inch, which I fear it will. I think that the cylinder must either be made larger, or make more than 60 strokes per minute; and I do not think the latter plan very advisable, principally because the rotative motion already turns too fast for the axle of the chaise, and that it will require more wheels than two to reduce the motion to the proper velocity. As to working-gear, stopping and backing, with steering the carriage, I think these articles perfectly manageable.

"The proper place for the engine will be behind the carriage, and to act upon the hind-wheels. Cokes must be used in place of coals, to prevent the disagreeable circumstances of soot and smoke; but there will be no avoiding the sulphureous air, which, when going before a gentle wind, will prove suffocating. The shaking of the carriage will supersede the necessity of poking the fire, but will be apt to waste the coals, by making the cokes fall through the grate before they are consumed. The shaking of the carriage will be apt to derange the joints of the cylinder, &c., and render them untight; but perhaps some remedy may be devised for this in construction.

"My original ideas on this subject were prior to my inven

tion of these improved engines, or before the crank or any other of the rotative motions were thought of. My plan then was to have two inverted cylinders, with toothed-racks instead of pistonrods, which were to be applied to two ratchet-wheels on the axletree, and to act alternately; and I am partly of opinion that this method may be applied to advantage yet, because it needs no fly, and has some other conveniences.

"From what I have said, and from much more which a little reflection will suggest to you, you will see that without several circumstances turn out more favourable than has been stated, the machine will be clumsy and defective, and that it will cost much time to bring it to any tolerable degree of perfection; and that for me to interrupt the career of our business to bestow my attention on it would be imprudent. I even grudge the time I have taken to write these comments on it. There is, however, another way in which much mechanism might be saved, if it be in itself practicable, which is, to apply to it one of the self-moving rotatives, which has no regulators, but turns like a mill-wheel by the constant influx and efflux of steam; but this would not abridge the size of the boiler, and I am not sure that such engines are practicable." The remainder of this letter is occupied with the consideration of the arrangements proper to be made with a third party, (not named, but then in the employment of Messrs. Boulton and Watt, and known to have been Mr. William Murdock), in case of his prosecuting the design, which he appears at that time to have warmly entertained, and urged, of making steam wheel-carriages for sale to the public, under a licence from his employers, or in partnership with them.

Again,* "I am extremely sorry that William] M[urdock] still busies himself with the steam-carriage. In one of my specifications I have secured it as well as words could do it according to my ideas of it; and if to that you add Symington's and Sadler's patents, it can scarcely be patentable, even if free of the gen

* 12th September, 1786.

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LOCOMOTIVE STEAM-CARRIAGES.

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prevent as much

eral specification in the Act of Parliament; for even granting that what I have done cannot secure it, yet it can act as prior invention against anybody else, and if it cannot be secured by patent, to what purpose should anybody labor at it? I have still the same opinions concerning it that I had; but to as possible more fruitless argument about it, I have one of some size under hand, and am resolved to try if God will work a miracle in favour of these carriages. I shall in some future letter send you the words of my specification on that subject. In the. meantime I wish W[illiam] could be brought to do as we do, to mind the business in hand, and let such as Symington and Sadler throw away their time and money, hunting shadows." And, a few days later,* "You are certainly wrong in your computation of 18 lbs. of water serving your steam-carriage an hour. At present, where engines are wrought by condensation, to exert the force of one horse requires 10 lbs. of coals and 1 cubic foot of water per hour; but if steam of double density is used, as must be the case where there is no condensation, it will take 20 lbs. of coals and 2 cubic feet of water for each horse-power. These are the present facts, and I suspect the age of miracles is past. I am glad, however, that William applies to his business." And, to Dr. Black,† “You know I have long had plans of moving wheel-carriages by steam, and I have even described them in one of my patents some years ago. I believe I shall make some experiments on them soon, but have small hopes of their ever becoming useful."

* 23d September, 1786.

† 5th October, 1786.

CHAPTER XXVI.

NEW LAMPS-GRAVIMETER-CAOUTCHOUC TUBES-ARITHMETICAL MACHINE-ARTICU LATED WATER-PIPE-MACHINE FOR COPYING SCULPTURE-ITS GRADUAL PROGRESS AND ITS PERFORMANCES-DATES AND EXTRACTS FROM MSS. CONCERNING IT— INTENDED SPECIFICATION OF A PATENT FOR ITS INVENTION-RELATIVE DRAWINGS -TIME EMPLOYED IN ITS OPERATIONS-PERFECTION OF THE WORK DONE.

By another of what may be called his mechanical recreations, practised soon after the date of the last of his steam-engine patents, Mr. Watt seems to have realised the idea, made classical by the story of Aladdin, of "New lamps for old." For the following letter to Mr. Argand,* famed for his manufactures of that sort, contains various ingenious suggestions on the subject of better reading-lamps than had before existed; and for a long time lamps were made at Soho on Mr. Watt's principles, which gave a light surpassing both in steadiness and brilliance anything of the kind that had appeared in those comparatively dark ages; and which, indeed, we have seldom, if ever, seen equalled by the elaborate contrivances so much vaunted in our own days of more general illumination.

"I have just seen some of Keir's lamps, but have not seen them tried; in my opinion, they will be found troublesome, and subject to be out of order; for the quality of the saline liquor must be adjusted to a drachm, otherwise they will not answer; besides, I should suspect that said liquor will have bad effects upon the oil, or upon the vessels containing it. I am sure they are clumsy, logger-headed things, top-heavy, and liable to be

overset.

* August 8th, 1787.

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