104 COEEELATION OF PHYSICAL FORCES. 



ing it with others ; the mind once possessed of the idea of a 

 fluid, soon invested it with the necessary powers and proper- 

 ties, and grafted upon it a luxurious vegetation of imaginary 

 offshoots. 



In what I am here throwing out, I wish to guard myself 

 from being supposed to state that the theory, historically 

 viewed, followed exactly the dates of the discoveries which 

 were effectual in changing its character ; sometimes a dis- 

 covery precedes, at other times it succeeds to a change in the 

 general course of thought ; sometimes, and perhaps most 

 frequently, it does both i. e. the discovery is the result of a 

 tendency of the age and of the continually improved methods 

 of observation, and when made, it strengthens and extends the 

 views which have led to it. I think the phases of thought 

 which physical philosophers have gone through, will be found 

 generally such as I have indicated, and that the gradual ac- 

 cumulation of discoveries which has taken place during the 

 more recent periods, by showing what effects can be produced 

 by dynamical causes alone, is rapidly tending to a general 

 dynamical theory into which that of the imponderable fluids 

 promises ultimately to merge. 



Commencing with electricity as an initiating force, we 

 get motion directly produced by it in various forms ; for in- 

 stance, in the attraction and repulsion of bodies, evidenced by 

 mobile electrometers, such as that of Cuthbertson, where 

 large masses are acted on ; the rotation of the fly-wheel, 

 another form of electrical repulsion, and the deflection of 

 the galvanometer needle, are also modes of palpable, visible 

 motion. 



It would follow, from the reasoning in this essay, that 

 when electricity performs any mechanical work which does 

 not return to the machine, electrical power is lost. It would 

 be unsuitable to the scope of this work to give the mathemati- 

 cal labours of M. Clausius and others here ; but the follow- 

 ing experiment, which I devised for making the result evi- 



