Blake on the Teeth of Cog-Wheels. “99 
piteh circles, (See Fig. 7.) “If the generating straight line 
‘be considered as a circle with an infinite diameter, the curve 
described on one ‘virelé® may be considered as the last of 
the exterior epicy clos) and that on the other as the last of the 
Arrmipsic Curves. 
Prov. 11.—If a describing point be made to move in a 
tine which joins the centres of two circles, and if at the same 
ime the circles be made to revolve in such a manner that 
