ON HYDRAULICS AS A BRANCH OF ENGINEERING. 163 



des Fonts et Chaussees, and Director of the Works of the 

 Canal I'Ourcq at Paris *. 



He is the author of several papers on the theory of running 

 waters, and of a valuable series of experiments on the motions 

 of fluids in capillary tubes. 



M. Coulomb had given a common coefficient to the two terms 

 of his formula representing the resistance of a fluid, — one pro- 

 portional to the simple velocity, the other to the square of the 

 velocity. M. Girard found that this identity of the coefficients 

 was applicable only to particular fluids under certain circum- 

 stances ; and his conclusions were confirmed by the researches 

 of M. Prony, derived from a great many experiments, which 

 make the coefficients not only different, but very inferior to the 

 value of the motion of the filaments of the water contiguous to 

 the side of the pipe. 



The object of M. Girard's experiments was to determine 

 this velocity ; and this he has effected in a very satisfactory 

 manner, by means of twelve hundred experiments, performed 

 with a series of copper tubes, from 1*88 to 2'96 millimetres in 

 diameter, and from 20 to 222 centimetres in length ; from which 

 it appeared, that when the velocity was expressed by 10, and 

 the temperature was 0, centigrade, the velocity was increased 

 four times when the temperature amounted to 85°. When the 

 length of the capillary tube was below that limit, a variation of 

 temperature exercised very little influence upon the velocity 

 of the issuing fluid, &c. 



It was in this state of the science that M. Prony (then having 

 under his direction different projects for canals,) undertook to 

 reduce the solutions of many important problems on running 

 water to the most strict and rigorous principles, at the same 

 time capable of being applied with facility to practice. 



For this purpose he selected fifty-one experiments which 

 corresponded best on conduit pipes, and thirty-one on open 

 conduits. Proceeding, therefore, on M. Giraird's theory of the 

 analogy between fluids and a system of corpuscular solids or 

 material bodies, gravitating in a curvilinear channel of indefinite 

 length, and occupying and abandoning successively the dif- 

 ferent parts of the length of channel, he was enabled to express 

 the velocity of the water, whether it flows in pipes or in open 

 conduits, by a simple formula, free of logarithms, and requiring 

 merely the extraction of the square root-f. 



• Essai stir le Mouvement des Eaux courantes : Paris 1804. Recherches 

 sitr les Eaux publiqiies, ^-c. Devis general du Canal I'Ourcq, ^-c. 

 + Memoires dfs Savans Ef rangers, 8fc. 1815. 



M 2 



