236 DISCOURSE ON THE STUDY 



of an almost insuperable nature to the regular de- 

 ductive application of the general principles of 

 mechanics to this subject, which arise from the 

 excessive intricacy of the pure mathematical en- 

 quiries to which its investigation leads. It was 

 Newton who set the example of a first attempt to 

 draw any conclusions respecting the motion of fluid 

 masses by direct reasoning from dynamical prin- 

 ciples, and thus laid the foundation of HYDRO- 

 DYNAMICS ; but it was not till the time of D'Alem- 

 bert that the method of reducing any question 

 respecting the motions of fluids under the action 

 of forces to strict mathematical investigation could 

 be said to be completely understood. But the cases 

 even now in which this mode of treating such ques- 

 tions can be applied with full satisfaction are few 

 in comparison of those in which the experimental 

 method of enquiry as already observed (189.) is 

 preferable. Such, for example, is that of the re- 

 sistance of fluids to bodies moving through them ; 

 a knowledge of which is of great importance in 

 naval architecture and in gunnery, where the re- 

 sistance of the air acts to an enormous extent. 

 Such, too, among the practical subjects which de- 

 pend mainly on this branch of science, are the use 

 of sails in navigation ; the construction of wind-mills, 

 and water-wheels ; the transmission of water through 

 pipes and channels ; the construction of docks and 

 harbours, &c. 



Nature of Solids in general. 

 (257.) The intimate constitution of solids is, in all 

 probability, very complicated, and we cannot be said 



