ON FORCE. 281 



out the assumption here made, M. Weber is obliged to suppose that 

 the molecules of diamagnetic bodies are surrounded by channels, 

 in which the induced molecular currents, once excited, continue 

 to flow without resistance."* Diamagnetism and Magne-crystallic 

 Action, p. 136-7. 



CHAPTER XVI. 



ON FORCE, f 



A SPHERE of lead was suspended at a height of 16 feet 

 above the theater floor of the Royal Institution. 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 of work 

 done in all these cases, as far as the raising of the weight 

 is concerned, would be absolutely the same. The work 

 done at one and the same place, and neglecting the small 

 change of gravity with the height, depends solely upon two 

 things; on the quantity of matter lifted, and on the height 

 to which it is lifted. If we call the quantity or mass of 

 matter m, and the height through which it is lifted h, then 

 the product of m into li, or m k, expresses, or is propor- 

 tional to the amount of work done. 



Supposing, instead of imparting a velocity of 32 feet a 



* In assuming these non-resisting channels M. Weber, it must be 

 admitted, did not go beyond the assumptions of Ampere. 



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



