740 
PROFESSOR 0. REYNOLDS ON CERTAIN DIMENSIONAL 
this threatened to prevent any satisfactory result being obtained. At last, however, 
by having steam on both sides, and repeatedly exhausting and refilling with air that had 
been passed slowly through drying tubes, 40 inches long, containing first sulphuric and 
then anhydrous phosphoric acid—for which I am indebted to the kindness of Dr. Hoscoe 
—the effect of vapour was all but eliminated, and consistent results were obtained over 
several trials, even when the sides of the steam and water were reversed. 
The differences of temperature. 
18. The steam used for heating the apparatus was obtained by boi lin g water in a 
glass flask which held about a gallon, enough to last for twelve hours at a time. The 
flask was fitted with a water safety-valve; so that the pressure of steam could not 
exceed about 8 inches of water. The flask was placed about 6 feet from the instru¬ 
ment, so that the heat from the gas flame did not produce any material disturbance 
or materially affect the mercury in the gauges. 
The cold water was direct from the main, and was found to be very constant in 
temperature, not varying throughout the experiments more than 23°—from 47° F. in 
February to 70° F. in July. 
In this way the tin plates (GG, fig. 1) which bound the gas chambers were respec¬ 
tively maintained at temperatures differing by less than 1° F. from the temperature of 
the steam (212°) and that of the water. 
The sides of the porous plate would not acquire the same temperatures as the steam 
and water, because the conduction through the porous plates would tend to equalise 
the temperature. Nor was there any means of ascertaining the exact temperatures 
other than by comparing the results obtained. But from these it appeared that there 
was considerable difference between the temperature of the surfaces of the porous plate 
and that of the opposite tin plate. A method of eliminating this difference has been 
found, and this will be explained with the results themselves. 
The porous plates. 
19. These, whether of biscuit ware, meerschaum, or stucco, were circular discs 
2 inches (53'0 millims.) in diameter. The rings FF which formed the chamber had a 
diameter of 1^ inches (38 millims.), and these limited the portion of the plate exposed 
to the passage of gas. The plates were of different thicknesses, the thinest being 
y- 6 th inch (1*5 millims.) and the thickest '44 inch (14'2 millims.). 
The results with air through porcelain plate No. 3 and meerschaum Nos. 1 and 2. 
20. After numerous experiments, commencing on January 23, with plates Nos. 1, 
2, and 3 of biscuit ware, the results of which, although there appeared to be a residual 
