90 CORRELATION OF PHYSICAL FORCES. 



which were effectual in changing its character; sometimes a 

 discovery precedes, at other times it succeeds a change in the 

 general course of thought ; sometimes, and perhaps most fre- 

 quently, it does both i.e. the discovery is the result of a ten- 

 dency 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 accu- 

 mulation 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 dyna- 

 mical theory into which that of the imponderable fluids pro- 

 mises ultimately to merge. 



Commencing with electricity as an initiating force, we get 

 motion directly produced by it in various forms ; for instance, 

 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, is also a mode 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 mathematical 

 labours of M. Clausius and others here ; but the following 

 experiment, which I devised for making the result evident to 

 an audience at the Royal Institution, will form a useful illus- 

 tration : A Leyden jar, of one square foot coated surface, 

 has its interior connected with a Cuthbertson's electrometer, 

 between which and the outer coating of the jar are a pair of 

 discharging balls fixed at a certain distance (about half an 

 inch apart). Between the Leyden jar and the prime conductor 

 is inserted a small unit jar of nine square inches surface, the 

 knobs of which are o - 2 inch apart. 



The balance of the electrometer is now fixed by a stiff 

 wire inserted between its knobs, and the Leyden jar charged 

 by discharges from the unit jar. After a certain number 

 of these, say twenty, the discharge of the large jar takes 

 place across the half-inch interval. This may be viewed 



