OP ARTS AND SCIENCES. 27 



the law followed by these lowerings, according to the diameter of the 

 orifice in communication with a current, and according to the velocity 

 of the latter." 



In the published records of the experiments relative to open canals,* 

 the results here anticipated are not included, and we are not informed 

 of the later conclusions of this able engineer. 



In the performance of my duties as engineer of the Essex Com- 

 pany, — a company controlling the water power of the Merrimack 

 River at Lawrence, Mass., — it has become important to interpret 

 with accuracy the indications of piezometers, and to determine the 

 circumstances affecting their reliability. To this end, a long series of 

 experiments has been made upon piezometers, having orifices of com- 

 munication of varying size and form, and through a wide range of 

 velocities. The results of these experiments are regarded as of impor- 

 tance to investigators in hydraulics, and are, through the liberality of 

 the officers of the Essex Company, now presented for their use. 



It is well known from the experiments of Venturi f that within a 

 short distance from the entrance of a pipe — a distance limited by the 

 influence of the contraction at the entrance — piezometers indicate a 

 pressure varying with their position, and widely different from that 

 which obtains after the section influenced by contraction is passed. 

 These phenomena are readily explained without attributing any dis- 

 crepancy between the pressure upon the sides of the pipe and the 

 indications of the piezometer. It is now, however, only necessary to 

 consider that portion of the pipe or conduit in which uniform motion 

 is established ; that is, a portion in which the particles of water move 

 parallel with the sides of the pipe with a velocity neither increasing 

 nor decreasing. 



Uniform motion then existing, the prominent facts to be determined 

 are whether the height of the piezometric column is lowered by the 

 cohesion of the water acting at the base of the piezometer or not ; 

 and whether or not the height of the column of still water indicates 

 with accuracy the height of the surface of the adjacent mass of moving 

 water in an open conduit. 



* Recherches Hydrauliques, entreprises par M. H. Darcy, Inspecteur-General 

 des Fonts et Chaussees : continuees par M. H. Bazin, Ingenieur des Ponts et 

 Chaussees. Premiere Partie : Recherches Experimentales sur l'Ecoulement de 

 l'Eau dans les Canaux de'couverts. Paris, 1865. 



t Tracts on Hydraulics. Edited by Thomas Tredgold, C. E. II. Venturi's 

 Experiments on the Motion of Fluids. Second Edition. London, 1836, page 

 136 et seq. 



