SEQUEL TO THE GENERALIZATION. 119 



science, the laws of extreme simplicity and gene- 

 rality from which the phenomena flow ; we cannot 

 doubt that the ultimate principles which we have 

 obtained are the true ones, and those which really 

 apply to the facts ; and yet we are far from being 

 able to apply the principles to explain or find out 

 the facts. In order to do this, we want, in addition 

 to what we have, true and useful principles, inter- 

 mediate between the highest and the lowest; 

 between the extreme and almost barren generality 

 of the laws of motion, and the endless varieties and 

 inextricable complexity of fluid motions in special 

 cases. The reason of this peculiarity in the science 

 of Hydrodynamics appears to be, that its general 

 principles were not discovered with reference to 

 the science itself, but by extension from the sister 

 science of the Mechanics of Solids ; they were not 

 obtained by ascending gradually from particulars 

 to truths more and more general, respecting the 

 motions of fluids; but were caught at once, by a 

 perception that the parts of fluids are included in 

 that range of generality which we are entitled to 

 give to the supreme laws of motion of solids. 

 Thus, solid and fluid dynamics resemble two edifices 

 which have their highest apartment in common, 

 and though we can explore every part of the former 

 building, we have not yet succeeded in traversing 

 the staircase of the latter, either from the top or 

 from the bottom. If we had lived in a world in 

 which there were no solid bodies, we should pro- 



