208 Mr. Faraday on a 



four bearings for the support of two axes, one on each side the 

 middle. The axes are small pieces of steel wire tapered at the 

 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 axes are made to revolve 

 in the following manner : A circular [copper plate about four 

 inches in diameter has three pullies 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 pullies, 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 sup- 

 port ; 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 ho- 

 rizontal 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 farther 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 pullies fixed on the 

 horizontal axes ; when, therefore, it is turned by the pro- 

 jecting pin, it causes the revolution of the axes. The varia- 

 tion in velocities is obtained by having the roller of different 

 diameters in different parts, and by having pullies 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 seven 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 



