A few clocks have been constructed with a view directly contrary to those described, in which simplicity of parts was as much studied as great variety of movements were in the others. Of the clocks of this simple structure, none have as yet exceeded that contrived by the celebrated Dr. Franklin : it shows the hours, minutes, and seconds, and yet consists of but three wheels, and two pi nions. The lowest wheel contains 160 teeth, and goes round once in four hours; it carries the hand on its axle, which points out both the hours and the mi nutes, as will be described ; and it turns a pallet above it of ten leaves, on the same axis with which is a wheel of 120 teeth, that gives motion to a pallet of eight leaves. The second hand is annex ed to the same axis with this latter pallet, as also the swing wheel, which carries 30 teeth, that gives motion to the pallets of an anchor escapement, and to its pen dulum that vibrates seconds. The dial of this clock is of a singular formation. The external circle on it contains 240 divi sions, numbered from 1 to 6J, in four successive notations. This circle shews the minutes : within it the hours are arranged in four concentric circles, or in a volute of four revolutions, along four radii, which form right angles with each other. By this arrange ment, while the point of the hand shews the minute, its side exhibits the hour ; or, more strictly speaking, shews that the hour is one of three; but so that it will hardly ever happen that any doubt will remain of which it may be, as there are four hours difference between the figures next to each other on the same radial line. A small circle is placed above the great one, and divided into 60 parts for the seconds. This clock was wound up by a line going over a pulley and ratchet on the axis of the great wheel, by which the weight was drawn up in the same manner as in the common wooden clocks. Many of these clocks have been made, which are found to mea sure time exceedingly well.
The small imperfection in this clock, of its leaving the uncertainty mentioned as to which of three hours it denotes, though so easily corrected by the judg ment, has given rise to some ingenious contrivances to obviate it.
That of Mr. Ferguson is best known, in which the hours were engraved on the face of the lower great-wheel ; the seconds on that of the upper or swing wheel ; and the minutes were shown in a fixed dial, outside all, through holes cut, in which certain small portions of the other two moveable dials were ex hibited; the minute hand was attached to the axle of the second great wheel, which contained 120 teeth, as well as the first great wheel ; the swing wheel had 90 teeth, the axis of the second great wheel carries a pinion of 10 leaves, and that of the swing wheel a pinion of 6 leaves. But this clock had several im perfections, from which Dr. Franklin's clock is free. The smallness of the teeth of the swing wheel caused the pendulum to describe smaller arcs than it should do ; the weight of the flattering, on which the seconds were engraved, load ed the axis of the wheel, so as to cause much friction in this part, which should be as free from it as possible, and there was a considerable difficulty in adjusting the hour plate so as to correspond with the minute hand.
Another very ingenious contrivance for the same purpose has been made in a clock, on Dr. Franklin's principle, in the possession of Mr. Patoureaux, clock maker, Wardour-street, to which the tubular pendulum, on Chandler's plan, before mentioned, is annexed. To the axis of the great wheel of this clock two concentric plates are annexed, the external one of which has a groove cut through it, along the line of a volute of four revolutions. This groove forms a trough, in which a metal ball is placed, part of which is seen through its ex cavations. As the plate and groove turn round, the ball rolls along the volute, still approaching nearer the centre as it proceeds ; and when at last it arrives at the centre, it falls into another trough, by which it is again conveyed to the ex ternal part of the volute ; the hours are engraved between the revolutions of the volute, and the minutes are marked on an external fixed circle, to which an in dex, annexed to the volute plate, points. We have not been able to discover who is the author of this ingenious invention. It is certainly a superior method to Fer guson's ; the moveable dial being in it annexed to the axle of least motion, where of course its weight is of least con sequence ; and the adjustment for the hours and minutes being performed in it at the same time. This clock is formed with a dead beat escapement, and is in tended for a regulator.
The description• of the parts of an eight day clock, moved by weights, in serted a little farther on, with refer ence to the annexed plate, may serve, with a little addition; to give an idea of the mechanism of a clock moved by a main spring.
The spring, by which a clock is moved, consists of a long flat plate of steel coiled up in a spiral form ; it is inclosed in a cylindrical box, to which its external ex tremity is attached, while its internal end is connected to a fixed axis, round which the spring-box revolves. As the strength of the spring is greater the more it is coiled up by the turning round the box, its action would be unequal in impelling the work of the clock ; and to remedy this inconvenience the fusee wheel has been contrived. The fusee consists of a conical barrel, round which an heliacal groove is cut, that receives a chain or catgut, previously wound round the spring box, by which, as it is turned round, it coils up the spring ; the groove receives the chain first near the base of the cone, and, as the barrel revolves, gradually brings it nearer the axis ; by this means, the stronger the spring is coiled up, the shorter is the lever by which it acts on the work ; and as it gradually uncoils and becomes weaker, on the contrary, the lever of action be comes longer.