SECT. ii. HEAT GENERATED BY IMPETUS. 27 



ten" feet, it will require four times the labour to raise 

 an equal weight to forty feet. If both these weights be 

 allowed to fall freely by their gravitation, at the end of 

 their descent, their velocities will be as one to two, that 

 is as the square roots of their heights, but the effect 

 produced will be as their masses multiplied by one and 

 four; but these are the squares of their velocities. 

 Hence impetus or vis viva is equal to the mass multi- 

 plied by the square of the velocity. Thus impetus is 

 the true measure of the labour employed to raise the 

 weights, and of the effect of their descent, and is entirely 

 independent of time. 



It is well known that iron becomes red-hot by per- 

 cussion or impetus. The atoms of the iron are thrown 

 into vibration, and these minute motions communicated 

 to the nerves produce the sensation of heat. Now the 

 mechanical labour required to raise the hammer to any 

 number of feet is equal to the weight of the hammer 

 multiplied by that number of feet ; but the impetus or 

 mechanical effect of the fall of the hammer is equal to 

 its mass multiplied by the square of the velocity, that 

 is to the vis viva : hence the quantity of heat generated 

 is proportional to the vis viva. The circumstances being 

 the same, if the mass be doubled the amount of heat is 

 doubled ; and if the velocity be doubled the amount of 

 heat is quadrupled. If the weight and the perpen- 

 dicular height through which a body has fallen be 

 known, the quantity of heat generated may be deter- 

 mined. The same amount of heat is generated by the 

 same amount of force, whatever that force may be, 

 whether impetus, friction, or any other. 



Dr. Thomson has put in a strong point of view the 

 quantity of heat that might be generated by percussion 

 or impetus. He computed that if by any sudden shock 

 the earth were arrested in its orbit, the heat generated 

 by the impulse would be equal to 11,200 degrees of the 



