PHYSICAL SCIENCE IN 1838. 1 7 



those who work in the light it is almost impossible to con- 

 ceive the difficulties encountered by those working in the 

 dark. So, looking back, after the general revolution in 

 philosophical thought caused by the recognition of the con- 

 servation of energy, it is very hard to arrive at a just estimate 

 of the work of those by whom it was effected. 



To the present student, endeavouring to enter fully into 

 the history of the discovery, difficulties are presented by 

 his own familiarity with expressions and terms, now part of 

 riis language, but which came into existence during the dis- 

 covery, by his familiarity with those facts the discovery of 

 which resulted in the generalization, and by his familiarity 

 with facts almost innumerable, previously unknown, but 

 which have been revealed as a consequence of the generali- 

 zation. 



The terms 'energy' and 'work* did not exist in the 

 language of science in their present significance. The vis 

 viva of a body, the product of the square of its velocity 

 multiplied by its mass, had since the time of Newton been 

 recognized as a mechanical quantity, and the term 'energy' 

 had been applied to the half of this quantity by Young. 

 On the other hand, 'work' — motion against resistance — 

 expressed as the product of the distance, multiplied by 

 the mean resistance overcome, although it was known to 

 express the half of the change in the vis viva which takes 

 place in a body moving against resistance, had never been 

 recognized in the schools of mechanical philosophy as a 

 fundamental measure of mechanical action, either as 'work' 

 or by any other name. 



Outside the schools of mechanical philosophy, engineers 

 engaged in constructing and using the steam engine had 

 long been led to recognize motion against resistance as the 

 C 



