350 HISTOEY OF MECHANICS. 



parts remain immovable, as if they were frozen; in this way he finds 

 a result agreeing with Torricelli's experiments on the velocity of the 

 efflux. 



We must allow that the assumptions by which this result is obtained 

 are somewhat arbitrary; and those which Newton introduces in 

 attempting to connect the problem of issuing fluids with that of the 

 resistance to a body moving in a fluid, are no less so. But even up to 

 the present time, mathematicians have not been able to reduce prob- 

 lems concerning the motions of fluids to mathematical principles and 

 calculations, without introducing some steps of this arbitrary kind. 

 And one of the uses of experiments on this subject is, to suggest those 

 hypotheses which may enable us, in the manner most consonant with 

 the true state of things, to reduce the motions of fluids to those 

 general laws of mechanics, to which we know they must be subject. 



Hence the science of the Motion of Fluids, unlike all the other 

 primary departments of Mechanics, is a subject on which we still need 

 experiments, to point out the fundamental principles. Many such 

 experiments have been made, with a view either to compare the results 

 of deduction and observation, or, when this comparison failed, to 

 obtain purely empirical rules. In this way the resistance of fluids, and 

 the motion of water in pipes, canals, and rivers, has been treated. 

 Italy has possessed, from early times, a large body of such writers. 

 The earlier works of this kind have been collected in sixteen quarto 

 volumes. Lecchi and Michelotti about 1765, Bidone more recently, 

 have pursued these inquiries. Bossut, Buat, Hachette, in France, have 

 labored at the same task, as have Coulomb and Prony, Girard and 

 Poncelet. Eytelwein's German treatise (Hydraulik), contains an 

 account of what others and himself have done. Many of these trains 

 of experiments, both in France and Italy, were made at the expense of 

 governments, and on a very magnificent scale. In England less was 

 clone in this way during the last century, than in most other countries. 

 The Philosophical Transactions, for instance, scarcely contain a single 

 paper on this subject founded on experimental investigations. 4 Dr. 

 Thomas Young, who was at the head of his countrymen in so many 

 branches of science, was one of the first to call back attention to this : 

 and Mr. Rennie and others have recently made valuable experiments. 

 In many of the questions now spoken of, the accordance which engi- 

 neers are able tc obtain, between their calculated and observed results, 



* Rennie, Report to Brit. ASMO. 



