MECHANICS. 81 



is confined to one single spot, and no sufficient time 

 is allowed for diffusing its action over the extent of 

 the door. 



The Rota Aristotellca, or Aristotle s Wheel. 



The outer circle of a wheel, while making a revo- 

 lution along a plane, describes thereon a right line 

 equal to its circumference. 



Now if this circle (called the deferent) carry along 

 with it another small circle concentric with it, having 

 no motion but what it receives from the deferent, 

 (like the nave of a coach-wheel carried along by the 

 wheel) ; this circle or nave, will, during the revolu- 

 tion, describe a line equal, not to its own circumfe- 

 rence, but to that of the wheel, because its centre 

 advances in a right line as fast as that of the wheel 

 does. 



The matter of fact is certain, but how it should be 

 seems a mystery. It has been attributed to the 

 sliding of the nave along the right line described by 

 the outer circle. 



The Descent of Bodies. 



A body falls by gravity precisely 16^ feet in a 

 second, and the velocity increases according to the 

 squares of the time : viz. 



In J" (quarter of a second) a body falls 1 foot. 

 |" (half a second) - ditto - 4 



1 second - - ditto - 16 



2 ditto - ditto - 64, 



3 ditto - - - ditto - 144 



The power of gravity at two miles distance from 

 the earth is four times less than at one mile.; at three 

 miles, nine times less, and so on. It goes on lessen- 

 ing, but is never destroyed, 

 o 



