the Foundations of Dynamics. 13 



thinking student who possesses some physical instincts but is 

 devoid of the artificial galvanic stimulus called Examination*. 



It may be that I myself err in an opposite direction, being 

 so interested in the muscles and the clothing that I forget 

 here and there a bone or two. If it be so it is a grievous 

 fault, and grievously shall I have to answer for it. It is not 

 a fault that I ever attempt to justify. Flabby and boneless 

 science is no science at all* 



In defining Energy, then, I first appeal to experience that 

 something in the popular sense energetic is often plainly pro- 

 duced in a body when work is done upon it ; e. g. when a bow 

 is stretched or a stone flung ; and I proceed to say that that 

 something is always produced, even when not obvious, and 

 that it has been called Energy because it frequently confers 

 upon the body possessing it the power of itself doing work. 

 I know well enough that the common definition runs, Energy 

 = power of doing work, but there is a difficulty about this 

 definition : plenty of energy has no power of doing work, or 

 at least no power that we can get hold of. Therefore I pre- 

 fer to give a name to the result of work done, whether it be 

 obviously energetic or not, and to justify the name energy by 

 appealing to the many cases where its possession does confer 

 a power of doing at any rate some work ; but in the definition 

 I make no statement that energy must necessarily be able to 

 do work, or must necessarily continue constant in quantity, 

 because such statements, so far as they are true, are part of 

 the law which is being led up to, not part of the definition. 



My definition of " energy " stands on all fours with the 

 customary definition of the potential function. It is a name 

 for the line-integral of a force, considered as a quantity that 

 can be stored. The line-integral of a force in action is work, 

 the result of it is energy. Or, otherwise : — The scalar product 

 of force and velocity is activity (sometimes called power) ; the 

 result of activity lasting a finite time is energy. 



Some conception of what energy is like can be gained by 

 appeals to familiar experience ; and the fact that it can be 

 stored more or less completely, and can ultimately reproduce 

 an activity similar to that which generated it, can also be 

 illustrated in a selection of cases. The law of energy asserts, 

 what cannot be so readily demonstrated, that in all cases it 

 can be stored without loss, except in so far as it leaks away, 

 or until it is discharged, in some kind of equivalent activity ; 

 and that even when thus dispersed or transferred or lost to 



* Valuable enough, however, in its proper place. I am not joining in the 

 outcry against examinations. They are a most useful and much needed 

 stimulus to right intellectual conduct, and partially replace the old belief 

 in purgatory. 



