52 PROCEEDINGS OP THE AMERICAN ACADEMY 



trough, by an amount which is a varying fraction of the pressure 

 which would produce the velocity with which the water approaches 

 the pipe. 



This fraction increases with the projection from the side, from zero 

 to forty -four one-hundredths, in the experiments in this trough ; and 

 in the experiments alluded to in a closed conduit uuder pressure, with 

 a smaller pipe having a thin end, it increased more rapidly with the 

 same distances from the side, and with greater distances increased 

 until the lowering exceeded the height which would produce the 

 velocity of the approaching water. 



The lowering of the piezometric column under circumstances like 

 these just presented confirmed Dubuat* in his conclusion that water 

 in motion pressed upon the sides of a conduit with a pressure less 

 than that due its depth, by the whole amount of pressure that would 

 produce its mean velocity, which conclusion Navier f controverted, 

 but which has been presented in works upon hydraulics quite gener- 

 ally until the publication of the results of M. Darcy, which in general 

 confirmed the position of Navier, but left in doubt the indications of 

 the piezometer ; but the experiments now presented show that with 

 currents flowing parallel with the side of a straight conduit, with orifices 

 having edges in the plane of the side and with passages normal thereto, 

 there is no lowering of the piezometric column, but that it indicates the 

 true height of the surface of the water in the conduit when in motion, 

 as well as when at rest. And we have a reliable datum plane, to 

 which observations in hydraulics may be referred. 



Note. Upon the Limits of Accuracy that may be obtained with 

 Piezometers. 



The experiments of the first group were united, because from careful 

 observation, made before any results were computed, the planes of the 

 orifices were regarded as satisfactorily in the plane of the side. No 

 deviation therefrom was perceptible in the light in which they could 

 be seen that enabled me to say they inclined one way or the other. 

 This light was good upon the side of the straight edge presented to 

 the eye ; but, looking down into the trough, no light could be seen be- 

 yond, and very slight variations could not be detected there, which could 

 be seen if the bearing surfaces were between the eye and the light. 



* Principes d'Hydraulique. Par M. Dubuat. Paris, 1816. Art. 439 and 453. 

 t Architecture Hydraulique. Par Belidor. Paris, 1819. Page 342. 



