534 Mr. H. Wilde on the Velocity 



condensing the air up to nine atmospheres, and also a Bour- 

 don's pressure-gauge of an improved construction, graduated 

 through every pound of the above pressure. The accuracy 

 of this gauge was tested in my presence by the constructors, 

 Messrs. Budenberg and Co., through the whole range of pres- 

 sure, by comparing its readings with a column of mercury 

 of equivalent height. For pressures of 15 pounds above, and 

 for pressures below the atmosphere, a mercurial gauge and 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 mer- 

 cury 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 in 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 w 7 as pierced with a circular hole two hundredths of an 

 inch in diameter. The size of the hole was accurately deter- 

 mined by means of a wire expressly drawn down to the above 

 diameter ; the wire being calibred by one of Elliott's micro- 

 meter-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. 



I have said that the capacity of the cylinder A was 573 

 cubic inches, which represents the same number of cubic 

 inches of air in the vessel at atmospheric pressure of 15 lb, 

 on the square inch ; and, generally, n times 573 cubic inches 



* Memoirs of the Manchester Literary and Philosophical Society, 

 vol. xxi. p. 104. 



