THE WAVE THEORY. 283 



gradually and painfully made the same discoveries, 

 proving the truth of his assertions and the exactness 

 of his demonstrations.' 



NOTE ON ENERGY. 



The passage in which Young introduces and defines 

 the terra energy is so remarkable that I venture to re- 

 produce it here. 



' The term energy may be applied, with great pro- 

 priety, to the product of the mass or weight of a body 

 into the square of the number expressing its velocity. 

 Thus, if a weight of one ounce moves with a velocity ot 

 a foot in a second, we may call its energy 1 ; if a second 

 body of two ounces have a velocity of three feet in a 

 second, its energy will be twice the square of three, or 

 18. This product has been denominated the living or 

 ascending force, since the height of the body's vertical 

 ascent is in proportion to it ; and some have considered 

 it as the true measure of the quantity of motion ; but 

 although this opinion has been very universally rejected, 

 yet the force thus estimated well deserves a distinct 

 denomination. After the considerations and demonstra- 

 tions which have been premised on the subject of forces, 

 there can be no reasonable doubt with respect to the 

 true measure of motion ; nor can there be much hesita- 

 tion in allowing at once that since the same force, con- 

 tinued for a double time, is known to produce a double 

 velocity, a double force must also produce a double 

 velocity in the same time. Notwithstanding the simpli- 

 city of this view of the subject, Leibnitz, Smeaton, and 

 many others, have chosen to estimate the force of a 

 moving body by the product of its mass into the square 

 of its velocity ; and though we cannot admit that this 

 estimation of force is just, yet it may be allowed that 

 many of the sensible effects of motion, and even the 

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