ON FORCE 391 



opposed in its flight by the resistance of gravity; but the 

 same holds true if the projectile be sent into water, mud, 

 earth, timber, or other resisting material. If, for example, 

 we double the velocity of a cannon-ball, we quadruple its 

 mechanical effect. Hence the importance of augmenting 

 the velocity of a projectile, and hence the philosophy of 

 Sir William Armstrong in using a large charge of powder 

 in his recent striking experiments. 



The measure then of mechanical effect is the mass of 

 the body multiplied by the square of its velocity. 



Now in firing a ball against a target the projectile, 

 after collision, is often found hot. Mr. Fairbairn informs 

 me that in the experiments at Shoeburyness it is a com- 

 mon thing to see a flash, even in broad daylight, when the 

 ball strikes the target. And if our lead weight be exam- 

 ined after it has fallen from a height it is also found 

 heated. Now here experiment and reasoning lead us to 

 the remarkable law that, like the mechanical effect, the 

 amount of heat generated is proportional to the product 

 of the mass into the square of the velocity. Double your 

 mass, other things being equal, and you double your 

 amount of heat; double your velocity, other things re- 

 maining equal, and you quadruple your amount of heat. 

 Here then we have common mechanical motion destroyed 

 and heat produced. When a violin bow is drawn across 

 a string, the sound produced is due to motion imparted 

 to the air, and to produce that motion muscular force has 

 been expended. We may here correctly say that the me- 

 chanical force of the arm is converted into music. In a 

 similar way we say that the arrested motion of our de- 

 scending weight, or of the cannon-ball, is converted into 

 heat. The mode of motion changes, but motion still con- 



