Mr. P. G. Tait on the History of Energy. 57 



weight of a pound through the space of a foot in a minute ; that 

 is, producing 550 foot-pounds of work per second. The unit, 

 however, which is most generally convenient is that which New- 

 ton's definition implies, namely, the rate of doing work in which 

 the unit of energy is produced in the unit of time. 



"Looking at Newton's words in this light, we see that they 

 may be logically converted into the following form : — 



" 'Work done on any system of bodies (in Newton's statement, 

 the parts of any machine) has its equivalent in work done against 

 friction, molecular forces, or gravity, if there be no acceleration ; 

 but if there be acceleration, part of the work is expended in 

 overcoming the resistance to acceleration, and the additional 

 kinetic energy developed is equivalent to the work so spent/ 



" When part of the work is done against molecular forces, as 

 in bending a spring ; or against gravity, as in raising a weight ; 

 the recoil of the spring, and the fall of the weight, are capable, 

 at any future time, of reproducing the work originally expended. 

 But in Newton's day, and long afterwards, it was supposed that 

 work was absolutely lost by friction.-" 



This shows that, so far as experimental facts were known in 

 Newton's time, he had the Conservation of Energy complete; 

 the cases of apparent loss by impact, friction, &c. were not then 

 understood. 



The opinion of James Bernoulli on a question of this nature 

 would undoubtedly be valuable, but he seems not to have noticed 

 Newton's remark. But I must protest against the allowing any 

 weight to that of John Bernoulli, who, while inferior to his 

 brother as a mathematician, was so utterly ignorant of the 

 principle in question as seriously to demonstrate the possibility 

 of a perpetual motion, founded on the alternate mixing of two 

 liquids and their separation by means of a filter. 



I take this opportunity of mentioning, with reference to Mr. 

 Monro's paper in the December Number of the Philosophical 

 Magazine, that in the very paper by Professor Thomson in which 

 the word "naturalist" is used (after Johnson) for Natural Phi- 

 losopher ; Dynamics is divided, as Mr. Monro suggests it should 

 be, into Statics and Kinetics. The same division is employed in 

 the pamphlet above quoted from. 



6 Greenhill Gardens, Edinburgh, 

 December 13, 1864. 



