60 COMMUNICATION OF MOTION IN MACHINES. 



scribed by a point of a thread which has been 

 wound round the wheel while it is uncoiled. 



A wheel which has teeth cut upon the circum- 

 ferences so as to project out in the plane of its 

 face, is called a spur wheel, (Plate 9.5. fig. 1.); and 

 when the projection of the teeth is at right angles 

 to the face of the wheel and parallel to the axis, 

 the wheel is called a crown, or contrate wheel, as 

 at B : C is a spur wheel. 



Sometimes the faces of the two wheels are in 

 the same plane, and consequently the axes parallel 

 as at fig. 1 ; sometimes the axes are at right angles 

 to each other, as fig. 2., one being a spur and the 

 other a contrate wheel : sometimes also the axes 

 are inclined to each other at various angles. 



A method much used for placing the teeth of 

 wheels, is by bevelling the edge, and cutting the 

 teeth on the bevel, by which means wheels can turn 

 each, though variously inclined, and the teeth have 

 also great strength. These are called bevelled 

 ivheelsy or bevel geer. 



Their principle consists in two cones rolling on 

 the surface of each other, as the cone A and B 

 revolving on their centres a b, a c (Plate 4. fig. 4); 

 if their bases are equal, they will perform their 

 revolutions in one and the same time; or any other 

 two points equally distant from the centre a, as 

 d 1, d 2, d 3, &c. will revolve in the same time as 

 f 1, f 2, f 3, &c. In the like manner, if the 

 cones a d ebe twice the diameters at the base d e, 

 as the cones afe are, then if they turn about their 

 centres when the cone a f d (Fig. 5. and 6.) has 

 made one revolution, the cone a d e will have 

 made but half a revolution j or when afe lias 

 made two revolutions, a d e will have made but 

 one, and every part equally distant from the centre 



