364 Dr. Gr. J. Stoney on the 



Such is an outline of the principles that underlie the dyna- 

 mical investigation of nature, which is the form of investiga- 

 tion that penetrates most deeply into its secrets. 



The dynamical investigation of nature being the most 

 complete from the physicist's standpoint, is of course to be 

 preferred to any other wherever it can be employed. Our 

 present knowledge of . astronomy, of rigid dynamics, of 

 elasticity, of hydrodynamics, are among its great achievements. 

 But there are other sciences in which we cannot penetrate 

 so close to the origin of things, but which are, nevertheless, 

 amenable to mathematical treatment onwards from a station 

 less deep-seated. In these we begin with happily chosen 

 equations, the truth of which we have not succeeded in tracing 

 to their dynamical source in nature, but the consequences of 

 which we can calculate and compare with what we observe to 

 occur. Of this kind are the exquisite theory of light which 

 was developed by M'Cullagh, and the enormous strides which 

 our knowledge of electricity has made within the last half 

 century, culminating in the marvellous electromagnetic theory 

 of light. 



In all such sciences we are greatly helped by mechanical 

 illustrations, which may be regarded as working models, that, 

 though they do not in the least profess to represent the 

 unknown dynamical condition which exists in nature, furnish 

 us with an apparatus which operates in ways that we can both 

 compute and conceive, and which produces results that, in 

 some important respects and within ascertainable limits, 

 follow laws the same as or analogous to those that prevail in 

 the part of nature which is illustrated by them. Of this kind 

 is that most useful and simplest wave-theory of light which 

 represents it by an undulation of mere transverse vibrations. 

 Of the same kind are the various attempts to represent the 

 " texture/' which must prevail in the luminiferous sether "*. 

 And somewhat akin to these are those instructive analogies 

 which can be traced out between different sciences, wherever 

 in both the same differential equation governs the progress of 

 events. Thus, when a current of electricity is turned on to a 

 circuit, the current penetrates the wire from its surface 

 towards its core, by the same law as that by which heat 

 would, by conduction, be carried inwards from the sur- 

 rounding dielectric — a process already familar to us, and 

 which, therefore, makes the sequence of events in the other 

 case easily conceived. 



Again it must be remembered that in every dynamical 



* See Scientific Proceedings of the Roy. Dubl. Soc. vol. vi. p. 392, or 

 Philosophical Magazine for June 1890, p. 467. 



