MARKS AN EPOCH IN SCIENCE. 105 



very anxious that this lecture should be published in its 

 •entirety as soon as possible. One paper refused to give 

 ■even a notice of it. After some discussion the Manchester 

 Guardian would, as a favour, print extracts to be selected 

 by themselves. This of course would not satisfy my brother. 

 I returned to the Manchester Courier ; and, after a long 

 debate, they promised to insert the whole as a special 

 favour to myself." 



Although thus fortunately preserved, the lecture was 

 lost sight of till 1884, so that Joule's generalization was not 

 before the scientific world. Then, after about five years, as a 

 •consequence of the recognition of Joule's discoveries, the 

 generalization was borne in upon the minds of those who 

 took up his work ; and, when expressed by Sir William 

 Thomson as the law of conservation of energy, it was 

 accepted as one of the fundamental laws of the universe. 



It thus becomes clear that this lecture marks an epoch 

 in the history of science, and that, besides the debt of 

 gratitude owing to Joule for making the discoveries which 

 rendered this generalization possible, there is owing to him 

 the further debt for having himself propounded the 

 generalization. 



In this lecture also Joule mentions, for the first time, 

 by way of illustration, his now accepted explanation of 

 Shooting Stars : — That they are meteorites rendered hot by 

 the friction they meet on encountering the atmosphere. 

 This explanation, to which Joule was directly led by his 

 experiments on the heat developed by the friction of fluids, 

 forms the subject of a future communication on Shooting 

 Stars. — Philosophical Magazine, 1 848. 



In the meantime, Joule had been making fresh deter- 

 minations of the mechanical equivalent of heat by 



