.S7A' II- 1 /./JAM SIEMENS, F.R.S. 185 



and magnetism, thereby creating what may justly be termed a new 

 science. Other names, including those of Seguin, Helmholtz, and 

 Tait, should not be passed over without mention. 



The popular volume entitled " Heat a Mode of Motion," which was 

 produced in 18G3 by Professor Tyndall, also did valuable service 

 by introducing to the larger scientific public a knowledge of this 

 most important new branch of science, and by terminating for 

 ever the view which had generally prevailed up to that time, 

 according to which heat and electricity were regarded as subtle 

 fluids or imponderable substances. According to present views, 

 all the agencies in nature may be defined as energy, varying only 

 in their outward manifestations, as heat or as electricity, as 

 chemical affinity or as mechanical effect, and presenting them- 

 selves as sensible or kinetic energy, or as dormant or potential 

 energy. 



Thus, when I lift a pound weight one foot high, muscular 

 kinetic energy is exercised, causing a certain consumption of the 

 potential energy resident in the muscle of the arm ; in suspend- 

 ing it, the force thus exerted becomes so much stored up, or 

 potential energy, which may at any time be called into requisition 

 for the accomplishment of various purposes. Thus, if attached 

 to a string passing over a pulley, it may be made to impart motion 

 to a train of wheels for driving a clock, or to accomplish any other 

 kind of mechanical work. Again, if allowed to drop from its 

 elevated position upon a plate of glass, it may produce the 

 mechanical effect of breaking the sheet of glass into fragments, 

 causing at the same time considerable sound, whilst, if allowed to 

 fall upon a sheet of lead, it will cause an indentation without 

 producing appreciable sound ; but if in this instance we could 

 have measured the temperature of the lead before it was struck, 

 and again immediately after, we should be able to detect a certain 

 increase of heat, the amount of which is absolutely determined by 

 Joule's equivalent ; thus, if the piece of lead were one pound in 

 weight and were equally heated throughout, by the shock of the 

 falling weight of 1 Ib. through 1 foot, 772 repetitions of the same 

 would produce heat sufficient to raise its temperature 34'13 Fahr., 

 which would again be equivalent to the heating of one pound of 

 water 1 Fahr., or to the unit of heat. Or our potential force of 

 cue foot-pound could be utilised to produce magnetism and an 



