DESCRIPTION OF PLATES. xvii 



PLATE XV. 



Fig. 189. The form of a wheel or pulley, on which a broad strap runs, the sur- 

 face being convex : the wheel which drives it is of a similar form, but its upper part 

 only is shown in the figure. P. 135. 



Fig. 190. The teeth of two wheels, formed into epicycloidal curves, acting on 

 planes : the dotted lines show the effective magnitude of the wheels. P. 135. 



Fig. 191. The teeth of two wheels, formed into involutes of circles, described by 

 uncoiling a thread from the dotted circles ; the point of contact of the teeth being 

 always in the straight line which touches both circles. P. 135. 



Fig. 192. Two surfaces formed into involutes of circles, revolving in contact with 

 each other, the equidistant lines, drawn on them, continuing to meet each other 

 throughout the revolution. P. 135. 



Fig. 193. The pinion A is of the kind called a spur wheel ; B is a crown wheel, 

 or a contrate wheel. P. 136. 



Fig. 194. The wheel and pinion are both bevilled ; the faces of the teeth being 

 directed to the point A. P. 136. 



Fig. 195. Two wheels a little eccentric, acting on each other. P. 137. 



Fig. 196. An eccentric contrate wheel, acting on a long pinion. P. 137. 



Fig. 197. A machine for cutting the teeth of wheels. A is the wheel, of which 

 the teeth are formed by the revolving saw B, turned by the wheel and pinion C, D, 

 by means of the handle E, while the frame which holds the saw, moving on hinges, 

 and resting on a spring, is depressed by the handle F, its place having been pre- 

 viously adjusted by the screw G. The large plate H I contains a number of concen- 

 tric circles, variously divided by points, into which the end of the spring I sinks at 

 each step, so as to fix the apparatus in the required position. P. 137. 



Fig. 198. A chronometer for measuring minute portions of time. The axis A B 

 being turned, either by the handle A or by the weight C, the balls D, E fly out, and 

 carry the weights F, G further from the axis ; in consequence of which the increased 

 effect of friction retards the motion, when it becomes too rapid. The barrel H is 

 turned in the mean time, with the axis, and is allowed to descend as the thread at I 

 is uncoiled, so that the point K, which is pressed against it by a spring, describes on 

 it a spiral, which is interrupted whenever the pin K is touched. P. 147. 



Fig. 199. The fusee of a watch or clock, the general outline of which forms part 

 of the hyperbola AB, in which the distance of each point from the axis CD is 

 inversely as its distance from the line D E. P. 148. 





