John Prescott Joule 23 



is never lost or gained in any of these transformations, 

 the total quantity in the universe remaining the same. 

 The simplest kind of energy is that of a body in motion, 

 and is measured by half the product of the mass and the 

 square of the velocity. If a heavy body be allowed to drop 

 from a height, it increases its velocity as it falls, and strikes 

 the ground with a certain amount of energy. If that energy 

 has not been created, it must have existed already when 

 the body was placed at the height from which it fell. Hence 

 we must recognize some form of energy which depends on 

 the gravitational attraction between the earth and the 

 body. This potential energy, as we call it, is being trans- 

 formed into the energy of motion (kinetic energy) as the 

 body falls. These are the two great subdivisions of energy. 

 If heat be not a substance, as was generally believed till 

 the middle of last century, but a form of energy, a definite 

 quantity of heat should be equivalent to a definite amount 

 of energy ; so that whatever the means by which we trans- 

 form mechanical work into heat, we ought always to get 

 the same amount. That this conclusion is correct was esta- 

 blished by Joule's researches. It forms our first law of 

 thermodynamics . 



John Prescott Joule 1 (1818-1889) began his scientific 

 career at the age of nineteen, and already six years later 

 he had established his position as one of the greatest 

 benefactors of the community. The characteristic quality 

 of mind which enabled him without aid and without en- 

 couragement to accomplish so much was his ability to fix 

 on the essential factors of a problem, and to verify his 

 ideas by accurate measurements. Inspiration came to him 

 from his own experiments ; his first ideas were hesitating 

 and sometimes wrong, but correcting them step by step, he 

 was led almost automatically to the final great discovery. 

 His cautious and strictly scientific procedure showed itself 

 at an age when an abundance of energy and originality so 

 often lead to ambitious speculations which are beyond the 

 powers of inexperienced youth. Joule published his first 



1 A valuable account of Joule's life and work, by Osborne Reynolds, 

 will be found in the Joule volume of tho Manchester Literary and 

 Philosophical Society. 



