742 
PROFESSOR 0. REYNOLDS ON CERTAIN DIMENSIONAL 
Table III.—Thermal transpiration of air by meerschaum plate No. 2 ('25 inch or 
6'3 millims. thick). Temperature of steam, 212° F. or 100° C.; temperature of 
water, 47° F. or 8° C. 
Mean pressure by vacuum 
gauge.* 
Difference of pressure 
by siphon gauge, March 15. 
Ratio of mean pressure to 
difference of pressure. 
inches. 
millims. 
inch. 
millhns. 
302 
764-5 
•25 
6096 
126 
12-9 
327-6 
•20 
5-080 
64 
8-53 
216-7 
•17 
4-318 
50 
370 
94-0 
•12 
3-048 
31 
2-0 
50-8 
•08 
2-032 
25 
0-88 
12-35 
•045 
1-143 
20 
0-5 
12-7 
•035 
0-889 
13-6 
Whether the fact that the thicker plate of meerschaum gave nearly three tunes the 
difference of either of the previous plates was due to the thicker plate maintaining a 
greater difference of temperature, or to some difference of texture in the thin plate, 
such as a flaw, has not been clearly determined, but it now appears probable that it 
was largely due to the first of these causes. 
With this plate lower pressures were for the first time tried, and Table III. shows 
these differences falling with the pressure. 
The ratio of the difference of pressure to the mean pressure, however, as is shown in 
the last column, increases as the pressure falls, and apparently is approximating to a 
constant value at lower pressures. This is according to Law III., Art. 9. 
From Law IV., Art. 9, it appears that this ratio should, as the pressure fell, have 
approximated to the ratio which the difference of the square roots of the absolute 
temperature on the two sides of the plate bears to the square root of the temperature 
on the side at which the pressure was measured. Assuming 1-i-13 to be this ratio, 
it would appear that there must have been considerable differences of temperature 
between the surfaces of the meerschaum and the side of the plate; but it also appeared 
probable that with still lower pressures the ratio might have been considerably lower. 
It would have been desirable to have carried the experiments to lower pressures, 
but at that time this was impossible as there was then no special means of reading 
the differential gauge; so that this had to be deferred until such a means was pro¬ 
vided. 
Hydrogen. 
21. In the meantime other gases v 7 ere tried. Owing to its lightness it was 
thought probable that hydrogen would at the higher pressure give a somewhat higher 
result than air. How much this might be the theory gave.no certain indication, for it 
depended on qualities of the gas which had not been determined. But at the lower 
pressure, according to Law IV., the difference of pressure should approximate towards 
the same value relatively to the absolute pressure. 
