738 
PROFESSOR 0. REYNOLDS ON CERTAIN DIMENSIONAL 
which escapes across, besides what passes through the porous plate, must pass through 
something like half an inch of india-rubber, or through a considerably greater thickness 
of wood. 
Communication with the chambers. 
13. The communication with the gas chambers is effected by means of the tubes KK, 
the outward ends of which are fitted with three and four branches respectively. 
By one of these branches the left chamber is connected with the open end of a 
mercurial vacuum gauge V or barometer tube, which measures the absolute pressure 
of this chamber. 
Another branch from the left chamber, and a branch from the right, are respectively 
connected with the two ends of a siphon tube S containing mercury, which acts as 
a differential gauge for measuring the difference of pressure in the two chambers. 
By means of the third branch from the left, and a second from the right, direct 
communication can be established between the chambers by turning a tap D. 
The third and fourth branches on the right are used to establish communication 
with a mercurial pump and to admit dry gas. 
These various connexions are shown in fig. la, Plate 47, which also shows the 
general arrangement of the apparatus. 
The connexions between the metal and glass tubes are made with thick india-rubber 
tubing, -|th inch bore and -§th inch external diameter; and the two taps D and P 
shown in the sketch are both of glass. 
The gauges. 
14. The vacuum gauge is an ordinary barometer tube about 32 inches long and 
L inch internal diameter, having its second limb sufficiently long to allow of the 
mercury standing level when the chambers were exhausted. 
The differential gauge is of glass tube about ^th inch internal diameter, it is 
altogether 30 inches long, so as to prevent the mercury being driven out of the tube 
by too great a difference of pressure. 
Before the mercury was put into this tube it was wetted with sulphuric acid. A 
small quantity of this remained and covered the mercury on either side, by means of 
which sulphuric acid the free motion of the mercury was secured, so that differences of 
pressure as small as xoo Fofb °f an inch of mercury caused it to move without the 
necessity of shaking. 
Reading the gauges. 
15 . As far as the vacuum gauge was concerned, there was no point to be gained by 
extreme accuracy in reading the absolute pressure of gas in both chambers, so that a 
scale attached to the gauge was found to answer all purposes. 
On the other hand the range of the experiments depended on the accuracy with 
