PROFESSOR O. REYNOLDS ON CERTAIN DIMENSIONAL 
762 
adjustment of the pressures on the right of the porous plate. On first opening the 
tap P, the mercury in the siphon gauge was displaced, and as equalisation was re¬ 
established the mercury re-assumed its level position, the instant of complete transpi¬ 
ration being that at which the mercury became level. 
The final adjustment of the mercury, however, was very slow, and it was not found 
possible, even with the cathetometer, to ascertain definitely the instant of complete 
equalisation. This threatened to be a difficulty, but it was finally overcome in a very 
simple manner. 
Instead of waiting for complete equalisation, the time was taken at which the 
equalisation had proceeded, until the residual excess of pressure to the left of the 
plate bore a certain relation to the initial absolute pressure—'002 was the proportion 
allowed. 
It will be seen that in this way the volume which passed, instead of being the 
volume for complete equalisation, was some definite proportion of this, and that the 
differences of pressure under which it passed were proportional to the initial difference 
of pressure, and hence the time occupied was the time of transpiration of equal 
volumes according to the previous definition. 
The manner of experimenting. 
37. The temperature of the room in which the diffusiometer was, having been read, 
the pump being full of mercury, and the taps D and P open so as to allow of complete 
equalisation through all the chambers of the instrument, the experiment commenced. 
The vacuum gauge was read ; this gave the initial pressure in the instrument. The 
position of the mercury on the left side of the differential gauge was then read with 
the cathetometer. 
From this reading was subtracted '001 of the reading on the vacuum gauge, i.e., the 
micrometer screw v~as turned through ten divisions for every inch pressure in the 
instrument. 
The vacuum gauge was then cut off by pinching the india-rubber tubing ; the taps 
P and D closed; one stroke of the pump was taken; a definite volume of air being 
thus drawn out of the flask, the pump was replaced so as to be full of mercury. Then 
at a given second, marked by a chronometer, the tap P was opened. A watch was 
then kept through the cathetometer, until the mercury in the differential gauge 
descended to fine in the cathetometer. As the mercury was still in motion, this instant 
was well marked by merely raising the eyes to the chronometer. 
The small losses of time (personal equations) between reading the chronometer and 
opening the tap, and reading the cathetometer and the chronometer, were determined 
as approximately equal to one second, which was accordingly subtracted from the time 
noticed. 
In one set of experiments, that of hydrogen through stucco, the time of equalisation 
was so small (between 20 and 30 seconds) that a fraction of a second became a matter 
