1856.] Professor T/iomsmi o?i Motive Power. 199 



WEEKLY EVENING MEETING, 



Friday, February 29. 



Sir Henky Holland, M.D. F.R.S. Vice-President, 

 in the Chair. 



Professor Wm. Thomson, F.R.S. 

 On the Origin and Transformations of Motive Power. 



The speaker commenced by referring to the term work done, as 

 applied to the action of a force pressing against a body which yields, 

 and, to the term mechanical effect prodtcced, which may be either 

 applied to a resisting force overcome, or to matter set in motion. 

 Often the mechanical effect of work done consists in a combination 

 of those two classes of effects. It was pointed out that a careful 

 study of nature leads to no firmer conviction than that work cannot 

 be done without producing an indestructible equivalent of mechan- 

 ical effect. Various familiar instances of an apparent loss of me- 

 chanical effect, as in the friction, impact, cutting, or bending of 

 solids, were alluded to, but especially that which is presented by a 

 fluid in motion. Although in hammering solids, or in forcing solids 

 to slide against one another, it may have been supposed that the 

 alterations which the solids experience from such processes con- 

 stitute the effects mechanically equivalent to the work spent, no 

 such explanation can be contemplated for the case of work spent in 

 agitating a fluid. If water in a basin be stirred round and left 

 revolving, after a few minutes it may be observed to have lost all 

 sensible or otherwise discernible signs of motion. Yet it has not 

 communicated motion to other matter round it ; and it appears as 

 if it has retained no effect whatever from the state of motion in 

 which it had been. It is not tolerable to suppose that its motion 

 can have come to nothing ; and until fourteen years ago confession 

 of ignorance and expectation of light was all that philosophy taught 

 regarding the vast class of natural phenomena, of which the case 

 alluded to is an example. Mayer, in 1842, and Joule, in 1843, 

 asserted that heat is the equivalent obtained for work spent in 

 agitating a fluid, and both gave good reasons in support of their 

 assertion. Many observations have been cited to prove that heat is 

 not generated by the friction of fluids : but that heat is generated by 

 the friction of fluids has been established beyond all doubt by the 

 powerful and refined tests applied by Joule in his experimental 

 investigation of the subject. 



