77'i 



PLATE XV. 



Fig. 189. The form of « wheel or pulley, on which a 

 'broad strap runs, the surface being convex : the wheel 

 wliich drives it is of a similar form, but its upper 

 part only is shown in the figure. P. 175. 



Fig. 190. The teeth of two wheels, formed into epi- 

 cycloidal curves, acting on planes : the dotted lines 

 show the effective nnagnitude of the wheels. P. 176. 



Fig. 191. The teeth of two wheels, formed into in-. 

 volutes 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. 176. 



Fig. 192. Two surfaces formed into involutes of cir- 

 cles, revolving in contact with each other, the equi- 

 distant lines, drawn on them, continuing to meet each 

 other throughout the revolution. P. 176. 



Fig. 193. The pinion A is of the kind called a spur 

 wheel ; B is a crown wheel, or a contrate wheel. P. 

 177. 



Fig. 194. The wheel apd pinion are both bevilled : 

 the faces of the teeth being directed to the point A. 

 P. 177. 



Fig. 195. Two wheels a little eccentric, acting on 

 each other. P. 178. 

 Fig. 196. An eccentric contrate wheel, acting on a 



ong pinion. P. 178 



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 ' previously adjusted by the screw G. The large 

 plate III contains a number of concentric circles, va- 

 riously 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. 178. 



Fig. 198. A chronometer for measuring minute por- 

 tions of time. The axis A B being turned, either by 

 the handle A or by the weight C, the balls'D, E fly oat, 

 and carry the weights F, G further ftdm 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, 

 tiescribes on it a spiral, which is interrupted whenever 

 the pin K is touched. P. 191. 



Fig. 199. The fusee of a watch or clock, the gene- 

 ral outline of which forms part of the hyperbola A B, 

 in which the distance of each point from the axis C D 

 is inversely as its distance from the line D E. P. 192. 



