14 FERGUSON'S LECTURES. 



LECT ascent. Therefore, the velocity which a body acquires 



x^-v^-x^ by falling, is sufficient to carry it up again to the same 



height from whence it fell : allowance being made for 



the resistance of the air, or other medium in which the 



body is moved. 



Thus, the body D in rol- 

 ling down the inclined plane 

 A B will acquire such a 

 velocity by the time it ar- 

 rives at B, as will carry it 

 up the inclined plane B C> 

 almost to C; and would carry it quite up to C, if the 

 body and plane were perfectly smooth, and the air gave 

 no resistance. So, if a pendulum were put into 

 motion in a space quite free of air, and all other resis- 

 tance, and had no friction on the point of suspension, 

 it would move for ever : for the velocity it had acquired 

 in falling through the descending parts of the arc, would 

 be still sufficient to carry it equally high in the ascending 

 part thereof. 



Center of The center of gravity is that point of a body in which 

 the whole force of its gravity or weight is united. 

 Therefore, whatever supports that point bears the weight 

 of the whole body : and whilst it is supported, the body 

 cannot fall ; because all its parts are in a perfect equili- 

 brium about that point. 



and line of An imaginary line drawn from the center of gravity 

 of any body towards the center of the earth, is called 

 the line of direction. In this line all heavy bodies 

 descend, if not obstructed. 



Since the whole weight of a body is united in its 



In, the above illustration, we have, for the sake of simplicity, sap- 

 posed that the body would pass through but one foot in the lirst 

 second, this however, is not the fact, as any heavy body in that 

 period of time, would have passed through about sixteen feet, and 

 continue to increase in Mie same ratio. The rule therefore is to 

 multiply the square of the time by sixteen, which will give the entire 

 number of feet through which the body lias fallen. 



