XVI 



ON FORCE 1 * 



A SPHERE of lead was suspended at a height of 16 

 feet above the theatre floor of the Royal Institu- 

 tion. It was liberated, and fell by gravity. That 

 weight required a second to fall to the floor from that 

 elevation; and the instant before it touched the floor it 

 had a velocity of 32 feet a second. That is to say, if at 

 that instant the earth were annihilated, and its attraction 

 annulled, the weight would proceed through space at the 

 uniform velocity of 32 feet a second. 



If instead of being pulled downward by gravity, the 

 weight be cast upward in opposition to gravity, then, to 

 reach a height of 16 feet it must start with a velocity of 

 32 feet a second. This velocity imparted to the weight 

 by the human hand, or by any other mechanical means, 

 would carry it to the precise height from which we saw 

 it fall. 



Now the lifting of the weight may be regarded as so 

 much mechanical work performed. By means of a ladder 

 placed against the wall, the weight might be carried up 

 to a height of 16 feet; or it might be drawn up to this 

 height by means of a string and pulley, or it might be 

 suddenly jerked up to a height of 16 feet. The amount 



1 A discourse delivered in the Royal Institution, June 6, 1862. 



(389) 



