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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 eleva- 

 tion ; 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 of work done in all these cases, as far as 

 the raising of the weight is concerned, would be abso- 



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



