286 Mr. Airy on the Forms of the Teeth of Wheels. — 
But the mechanical action and the motion would be dreadfully 
irregular. 
I am informed by engineers, that this question is now little 
more than one of mere curiosity. In consequence of the very 
extensive use of iron, where wood was formerly employed, the 
teeth of wheels are now made so small, that it is of little conse- 
quence whether they have, or have not, the exact theoretical form. 
Almost all teeth are now made with plane faces passing through 
the axis of the wheel, and are expected to wear themselves in 
a short time into proper forms. This is the case with nearly all 
the modern iron wheels that I have examined; in the wheels of 
clock and watch-work, some attention to the figure is however 
thought necessary. 
G. B. AIRY. 
Trinity CoLLecE, = 
April 30, 1825. 
