Canon Moseley on the steady Flow of a Liquid. 349 



neously, which is unattainable with spring manometers ; for the 

 air-gauge showed a diminution of pressure of about \ pound 

 when the steam-whistle of the locomotive-boiler to which it was 

 attached was sounded, whereas no motion could be detected in 

 the spring and piston manometer. 



The price of the new manometer being no higher than that of 

 the spring manometer, but permitting greater certainty and ac- 

 curacy in the reading, it is hoped that it may be found to be an 

 accurate and durable gauge for steam-engines of every descrip- 

 tion, as well as for measuring the pressure of air in ventilation, 

 and in the vacuum-pans used in sugar manufactories. 



XLV. On the steady Flow of a Liquid. By Henry Moseley, 

 M.A.,D.C.L., Canon of Bristol, F.R.S., Corresponding Mem- 

 ber of the Institute of France, fyc. 



[Continued from p. 197. J 



wi 

 IT is by experiment that the value of 7 or of — in the equation 



y = v €~v r has been shown to be approximately constant. It 

 results, as shown in Table I., from the approximate constancy in 



M. Darcy's experiments of the fractions (— J and (_ ? J , 



and from the near accordance with experiment of the values of v 

 for different values of r calculated on the supposition of the con- 

 stancy of y. I propose now to investigate the value of 7 theo- 

 retically. 



The motion of the Liquid in the pipe is dependent on that in the 

 reservoir. 



A film of liquid flowing from a reservoir through a pipe may 

 be considered as a cylindrical prolongation of a vase-like liquid 

 film which begins in the reservoir, and which, contracting its ex- 

 ternal dimensions and increasing its thickness from the surface as 

 it descends, bends its neck to pass into the pipe. The motion of 

 the liquid which forms this vase is continuous. In the reservoir 

 it is subject to the same kind of resistances as in the pipe; and 

 the conditions of its motion in the pipe can only be fully de- 

 termined when those in the reservoir shall have become known. 

 U 2 * represents the work expended on the various resistances 

 opposed to the descent of the liquid in the reservoir and to 

 its passage from the reservoir into the pipe ; and this is one of 

 the terms of equation (2) which are neglected in equation (7) 

 and in equation (12). I am about to show that, by assigning a 

 proper value to 7 in each case, the error arising from this omis- 

 sion may be corrected. U, is another of the terms of equation 

 * Phil. Mag. September 1871, p. 186. 



