extremities; each has upon it a little roller or disc of soft wood, which, though it can be moved by force from one part of the axis to another, still has friction sufficient to carry the latter with it when turned round. These axe are made to revolve in the following manner:—A circular copper plate about 4 inches in diameter has three pulleys of different diameter fixed upon its upper surface, whilst its lower surface is covered with a piece of fine sand-paper attached by cement. A hole is made through the centre of the plate and pulleys, and guarded by brass tube, so fitted as to move steadily but freely upon an upright steel pin fixed in the middle of the centre wooden support; hence when the plate is in its place, it rests upon the two rollers belonging to the horizontal axes, whilst it is rendered steady by the upright pin. It can easily be turned round in a horizontal plane, and it then causes the two axes with their rollers to revolve in opposite directions, and the velocities of these can be made either equal to each other, or to differ in almost any ratio by shifting the rollers upon the horizontal axes nearer to, or further from the centre of the stand.
To produce motions of the axes in the same direction, an aperture was cut in the lower part of the upright board; a roller, turned for it, which loosely fitted within the aperture; and a steel pin or rod passed as an axis through the roller. The roller hangs in its place by endless lines made of thread, passing under it, and over little pulleys fixed on the horizontal axes; when, therefore, it is turned by the projecting pin, it causes the revolution of the axes. The variation in velocities is obtained by having the roller of different diameters in different parts, and by having pulleys of different dimensions. This description will be easily understood by reference to the figures.
This apparatus had to carry wheels either with cogs or spokes; which was contrived in the following manner:—The wheels were cut out of cardboard, were about 7 inches in diameter, and were formed with cogs or spokes at pleasure. A piece of cork, being the end of a phial cork, about the tenth of an inch in thickness, was then fastened by a little soft cement to the middle of the wheel, and a needle run through both, and then withdrawn. These wheels could at any time be put
