CORRELATION OF HEAT AND MOTION. 69 



motion is suddenly arrested, and its temperature as suddenly 

 raised. That which was motion has become heat, and the quan- 

 tity of heat shared between the bullet and the target is exactly 

 proportional to the previous velocity of the bullet, or to the 

 quantity of its motion that has been arrested. Again, in exact 

 proportion to the diminution of motion in the working parts of the 

 steam-engine, so is the amount of heat developed by the friction 

 of the drill upon the piece of metal. For not only are heat and 

 motion convertible into one another in a general way, but they 

 are convertible in a manner which admits of accurate measure- 

 ment. By suitably contrived experiments it may be shown, in 

 every instance, that so much heat developed is equivalent to so 

 much motion arrested, and conversely that so much motion 

 generated is equivalent to so much heat absorbed. It has been 

 ascertained for instance, more particularly by Mr. Joule, that the 

 force of a pound weight falling through 772 feet is exactly equal 

 to the amount of heat which a pound of water will give out in 

 cooling one degree Fahrenheit ; or, in other words, that the heat 

 developed by the arrest of the motion of a pound weight falling 

 through 772 feet would raise the temperature of a pound of 

 water one degree, and that the heat given out by the cooling of a 

 pound of water one degree would lift a pound weight to the height 

 of 772 feet. Heat and motion, therefore, are in every case inter- 

 changeable for one another according to a definite standard of 

 equivalency. > * -" 



(72.) Now, let me direct your attention to some simple illus- 

 trations of motion ; and, first of all, to the direct motion of a 

 projectile hurled from the arm of a man. Upon seeing the rapid 

 flight of a dart or javelin through the air, we recognise imme- 

 diately that the motion of the javelin did not originate in itself, 

 but was impressed upon it by some external force. We never 

 think of attributing its motion to the exercise of any peculiar 

 javelin force, but to the action of the muscular force by which it 

 was originally projected. We know that the momentum of the 

 javelin is exactly proportionate to the amount of force exerted in 

 hurling it, and that when it strikes some distant object, the blow 



