150 



MR. H. WILDE ON THE VELOCITY 



/ 

 / 



a Bourdon's vacuum-gauge were employed, the readings 



of which were compared with each other : 30 inches of 



mercury were considered equal to one atmosphere, and 2 



inches of mercury to one pound of pressure. The upper 



part of the glass tube of the mercurial gauge was fitted 



with a brass cap and screw-stopper, so that it could readily 



be used as a pressure-gauge, or as a vacuum-gauge when 



required. The discharging arrangement on the cylinder 



A consisted of a stopcock and union for securing a thin 



plate, through which the discharge was made. The orifice 



jU the plate opened as required, either directly into the 



atmosphere or into the end of a short iron tube two and 



a half inches internal diameter, communicating with the 



bottom of the cylinder. The thin plate was a small disk 



of tinned iron, three quarters of an inch in diameter and 



one hundredth of an inch in thickness. The centre of the 



disk was pierced with a circular hole two hundredths of an 



inch in diameter. The size of the hole was accurately 



determined by means of a wire expressly drawn down to 



the above diameter; the wire being calibred by one of 



Elliott's micrometer-gauges, divided into thousandths of an 



inch. The hole in the plate was enlarged so as to fit 



tightly the gauged wire, and the burrs on each side of the 



hole were carefully removed, as this small amount of 



projection, as Dr. Joule has shown ^, exercises a notable 



influence on the rate of discharge through apertures in 



thin plates. 



The general reasonings, and the inferences drawn from 

 the experiments to be described, are based on Boyle and 

 Mariotte's law of the density of a gas being as the pressure 

 directly, and the volume as the pressure inversely for 

 constant temperatures. 



* Memoirs of the Manchester Literary and Philosophical Society, vol. xxi. 

 p. 104. 



