418 
LOUD KAYLEIGH ON THE LAW OF THE PRESSUEE OF GASES 
nected below with one reservoir of air and above with another reservoir, or with a 
vacuum, the two gauges should reach their settings simultaneously, or at least so 
nearly that a suitable correction may be readily applied. For brevity we may for 
the present assume precise similarity. If now the two gauges be combined in series, 
so tliat the low-pressure chamber of the first communicates with the high-pressure 
chamher of the second, the combination constitutes a gauge suitable for measuring a 
doubled jjressure. 
The Manometers. 
Tlie construction of the gauges is modelled upon that used extensively in my 
researches upon the density of gases, so far as the j^rinciple is concerned, although of 
course the details are very different. In fig. 1 A and B represent 3/4 size the 
lower and upper chambers. As regards the glass-work, these communicate hy a 
short neck at D as well as by the curved tube ACB. Through the neck is carried 
the glass measuring-rod FDE, terminating downwards at hoth ends in carefully 
prepared points E, F. The rod is held, at D only, with cement, which also com¬ 
pletely blocks up the passage, so that when mercury stands in the curved tube the 
upper and lower chambers are isolated from one another. The use of the gauge is 
fairly obvious. Suppose for example that it is desired to adjust the pressure of gas 
in a vessel communicating with G to the standard of the gauge. Mercury standing 
in C, II is connected to the pump until a vacuum is established in the upper chamber. 
From a hose and reservoir attached below, mercury is supplied through I until the 
point F and its image in the mercury surface nearly coincide. If E coincides with its 
image, the j^ressure is that defined ; otherwise adjustment must be made until the 
points E, F both coincide with their images, or as we shall say until both mercury 
surfaces are set. Tlie pressure then corresponds to the column of mercury whose 
height is the length of the measuring-rod between the points E, F. The verticality 
of E F is tested with a plumb-line. 
The measuring-rods appear somewhat slender; but it is to be remembered that the 
instruments are used under conditions that are almost constant. So far as the 
comparison of one gas with another is concerned, the qualification “ almost ” may 
indeed l)e omitted. The coincidence of the jioints and their images is observed with 
the aid of four magnifiers of 20 millims. focus, fixed in the necessary positions. 
General Arrangement of Apparatus. 
In fig. 2 is represented the connection of the manometers with one another and 
with the gas reservoirs. The left-hand manometer can be connected above through F 
with the pump or with the gas supply. The lower chamber A communicates with 
the upper chamber D-of the right-hand manometer and with an intermediate 
