pears from a letter of Mr. Watt in 1791, in which it is incidentally mentioned that Mr. Rennie, "who is in considerable fame, and, I suppose," says Mr. Watt, "as well paid as any of his standing, has two guineas a-day when employed as an engineer;" in addition, however, it is to be presumed, to his travelling expenses and other "costs and outlays," which was not the case with the smaller rate of pay of Mr. Watt in 1770. t CHAPTER XVI. PHILOSOPHICAL PROBLEMS-TIME-PIECES-MICROMETER AND DIVIDING SCREW-SUR- BESIDES all his surveying and civil-engineering, and the manifold alterations he devised in both his condensing and his wheel engines, Mr. Watt bestowed, during the years of which we have now been speaking, in concert with his friend Small, a good deal of thought on various other ingenious mechanical contrivances, which supplied pleasant amusement to their inventive and reflective brains. "We have abundance of matter to discuss," says the great engineer; though the damned engine sleep in quiet!"* "The French, you know," says Small, "offer large præmia for time-keepers. Were I idle, I should try to win one of these. But physic exhausts my whole faculties, and pays but indifferently. I am so made that I suffer no fatigue from thinking ever so long and attentively on a subject in which I can get forward; but if I am absolutely puzzled, and see no clue, my head turns round, and I speedily become more tired than a galley-slave. Physic very fortunately furnishes abundance of these profitable points."† "I have perfected my clock with one wheel of nine inches diameter, which is to tell hours, minutes, and seconds, and strike, and repeat, and be made for thirty shillings."* And again, "My clock of one wheel, that shows hours, minutes, and seconds, and strikes the hours and repeats them, is nearly finished. The striking and repetition are good, the rest is gimcrack."† “You wrote me before," says Mr. Watt, "of your clock with one wheel. Did I ever mention to you a striking part, regulated by a balance pendulum with live scapement, which had only one wheel?" But some months later,§ "Everybody," says Small," is too much engaged for the prosecution of schemes, so that even my clock is not prosecuted, and I have only one, which I cannot send to you ;"—I have just ordered a pendulum clock to be made with no wheel at all;"|—and, "when my clock with one wheel," he afterwards adds,¶ "was finished, I found it too complicated, and have now got one with no wheel, and only one sector with seventy-five teeth. It strikes, repeats, shows hours, minutes, and seconds, and goes eight days, with the usual descent of the weight. This is to be ranked in mechanics as riddles and rebuses are ranked in poetry." "As On which comes this comment of the sagacious Watt :to clocks, I do not fully conceive how you can make yours go eight days with the ordinary descent of the weight, unless by pulleys or something equivalent, which would only be a quibble upon a wheel; "**—thus answered by the inventor :††-" there is no quibble in my clock, and we have now found a tolerable workman for the execution of it. One is now making, which will show with much more accuracy than any other clock has hitherto done, the spheric phenomena relating to astronomy, sidereal and mean time, hours, minutes and seconds, with only one wheel and one It will also strike and repeat the hours. The wheel has 72 teeth and the sector 75 only; "—" I have had a new scapement sector. made for watches, of such marvellous virtue, that if the maintaining power is quadrupled, or decupled, the number of the vibrations will be lessened, but not above ten in twenty-four hours." * And-"I have taken out a patent for improvement on clocks and other time-pieces, and want you vastly to help me to draw up the specification, which must be given in soon. I have made a churchclock, consisting only of one wheel of 126 pins, and one sector of 75 pins, and a hammer, with a scapement for the sector and another for the hammer. It strikes the hours, shows hours, minutes, and 20th parts of minutes, and goes eight days. And I have given drawings of a pocket-watch, which is to consist of one wheel and two sectors, and is to show hours, minutes, and seconds, and to have only 142 pins in the whole watch, and to have no chain nor fuzee, the kind of scapement rendering them unnecessary. But I have had hitherto villainous bad workmen. The axis of the pendulum of my clock is a cylinder, and rolls upon curves, which render the vibrations isochronous, and it has two rolls for one impulse."† “A patent,” he again writes, "has been taken out for my clocks and watches, and there is reason to hope they may become an article of commerce. I am ready to make over to you all my interest in the patent, provided that can be done so as to benefit all concerned, which, if you can be established in this country, might be the case." "As to gimcracks," writes Mr. Watt,§ "I have contrived a a new micrometer, made by drawing two converging lines upon glass. I believe from trial it will answer. I mentioned a dividing-screw; it has a wheel fixed upon it with 150 teeth and only 14 inch diameter: it is moved any portion of a turn or number of turns by a straight-line rack, the teeth of which fit it, without shake, and is moved by the hand or foot. It divides distinctly an inch into 400 equal parts." Of the micrometer, we shall soon give a full history. "As to your doubts about the screw," he writes MICROMETER AND DIVIDING-SCREW. 200 66 1000 185 to Small, "I intend to annihilate them when I see you :"-" my dividing-screw can divide an inch into 1000 tolerably equal and distinct parts on glass; "—“I had occasion to use my last dividingscrew for the first time the other day. It divided 9 inches into 20ths, and did not err the 1th of an inch in the whole 9 inches. I did not find that there was the least inequality among the divisions, though I subjected them to the most severe trial, and I have found a way by which I can divide a foot into ths of an inch without erring aboveth of an inch in the whole length, and the divisions shall be equal among themselves; so that I reckon that. machine exceeding near perfect, and find it very useful, as it saves much needless compass work, and, moreover, can divide lines into the ordinates of any curve whatsoever." "I rejoice in all your improvements," replies Dr. Small,* but have many optical difficulties that lessen my confidence in observations made with the most accurately divided instruments. For example, no optical instrument hitherto constructed, catoptric or dioptric, or catadioptric, produceth an exact copy of any object; so that all the visible points of every object of sensible apparent diameter are represented in the field of the instrument in situations in relation to each other very different from what they ought to occupy, &c., &c. The unsteady refractions of light passing through the atmosphere are also vile things; not those mentioned by astronomers only, but others I will show you when we meet.” "I am making a new surveying quadrant by reflection," writes Mr. Watt, "having the uses of a semicircle as taking angles to 180°; the principle, that of Bird's octant, in which the objects are only once reflected. In this I am making, the fixed glass stands at 45° to the first radius; and by shifting the place of the eye, the head is never in the way. I am going to make another altogether of glass, with nonius of the same." "I On which Dr. Small observes :-" I remember to have much admired your schemes about improving instruments for measuring angles by reflected light. Dollond has lately, as I have heard, * 29 March, 1774. + 17 January, 1773. |