On Coy or Toothed Wheels. 14] 
AB, will be traced the epicycloid d,e, fg, s,h, 
of which the circle ABF is called the base, 
and D the generating circle. Thus then the 
wheel to which the teeth are to belong is the 
base of the curve, and the wheel to be acted 
upon is the generating circle; but it must be 
understood that those wheels are not estimated 
in this description at their extreme diameters, 
but at a distance from their circumferences 
sufficient to admit of the necessary penetra- 
tion of the teeth; or, as M. Camus terms it, 
where the primitive circles of the wheels touch 
each other, which is in what is called in this 
country the pitch line. 
_, Now it has been long demonstrated by ma- 
thematicians, that teeth constructed as above 
would impart equable motion to wheels, sup- 
posing the pins 7, 2, t, &c. indefinitely small. 
This point therefore need not be farther in- 
sisted upon. 
_ So far the theoretic view is clear; but when 
we come to practice, the pins 1, 7, ¢, previ- 
ously conceived to be indefinitely smali, must 
have strength, and consequently a consider- 
able diameter, as represented at 1, 2; hence 
we must take away from the area of the curve 
a breadth as at v and x = to the semidiame- 
ter of the pins, and then equable motion will 
continue to be produced as before. But it is 
