PROCEEDINGS OF THE AMERICAN ACADEMY 



II. 



EXPERIMENTS UPON PIEZOMETERS USED IN 

 HYDRAULIC INVESTIGATIONS. 



By Hiram F. Mills, Civil Engineer. 



Presented April 10, 1878. 



In making experiments upon water flowing in pipes and in open con- 

 duits, it is most convenient to measure its pressure against the side of 

 the' pipe or conduit, and its depth in the conduit, by means of small 

 tubes extending through the side, normal to its surface, and communi- 

 cating with vertical columns of water, contained in glass tubes or in 

 small reservoirs. 



Such columns of water, used to measure pressure, are called 

 piezometers. 



The question has arisen : Do they indicate the actual pressure 

 against the side of the pipe, of the water when in motion, or do they 

 indicate the actual height of the surface of the moving water in the 

 conduit ? 



M. Darcy, in his great work * on the movement of water in pipes, 

 says (page 217) : " Indeed, manometers do not indicate the entire head 

 of a conduit at the points where they are adjusted, but this head 

 diminished by a certain height, the diminution being due to the velocity 

 of the fluid at the base of the piezometers : the water, by its cohesion, 

 acts upon the manometric column, whose height it lowers." 



Again, he says : " When one of the manometers was placed upon 

 the cylindrical reservoir, where the velocity of the fluid was very slight 

 relatively to that of the water in the pipe, we see that in like circum- 

 stances the lowering by suction of the manometer upon the reservoir 

 should be less than the lowering by suction of the manometer upon the 

 pipe. . . . There was then a rectification to be made, but I have not 

 at this time the means of making it. In the experiments relative to 

 open canals, with which I am now occupied, I shall seek to determine 



* Recherches Experimentales relatives au Mouvement de l'Eau dans les 

 Tuyaux. Par Henry Darcy, Inspecteur-Ge'neral des Ponts et Chausse'es. Paris, 

 1857. 



