21 
of mercury of equivalent height. For pressures of 151bs. 
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 mercury to one lb. 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 disc of tinned iron 
three-quarters of an inch in diameter, and one-hundreth of 
an inch in thickness. The centre of the disc was pierced 
with a circular hole two-hundreths of an inch in dimaeter. 
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. 
I have said that the capacity of the cylinder A was 573 
cubic inches, which represents the same number of cubic 
* Memoirs of the Manchester Literary and Philosophical Society, vol. 
xxi., p. 104, 
