74 
PHYSICS: C. BARUS 
Proc. N. A. S. 
In the experiments fringes of 1/2 scale part were installed. In separate 
experiments immediately after closing the cock C, a half turn of the screw 
produced a displacement of 8.3, 8.0, 8.0, 8.5, 8.5 scale parts, as the average, 
therefore 16.4 scale parts per turn or about 33 fringes per turn. This 
agrees as closely as may be expected with the number computed. 
The pressure increment per turn of screw is dp — n/2 cm. of mercury 
or per turn of screw about 10 ~ 3 cm. Per fringe, therefore, 3 X 10 ~~ 5 cm. 
of mercury as anticipated. A range of about 2 or 3 turns of screw was 
possible with each fringe, i. e., the range of pressure measurement should be 
from 3 X 10~ 5 to 3 X 10~ 3 cm. of mercury. 
Experiments of the same kind were made in great variety. There is 
no difficulty in using much larger fringes so that 3 X 10 ~ 6 cm. of mercury 
should be appreciable. By exhausting both sides of the U-tube the ap- 
paratus becomes a vacuum gauge. I did not, however, attempt much 
work with it as the present apparatus was not well adapted for the purpose. 
Air Thermometer. — If the cock C is permanently closed, the air space 
A becomes the bulb of an air thermometer of approximately constant 
volume. In this way the heat produced by the rays of light L' may be 
measured. In a variety of experiments of the kind, the mean result was 
about 10 scale parts or 20 fringes in a lapse of 210 seconds. If r denotes 
absolute temperature, the intrinsic equation may now be written 
dp dv _ dr 
~p + v ~ 7 
which reduces as above to 
dh dh _ dr 
76 + 2H " 7' 
Thus if r = 300° 
dT = " \{k + ar) - 10-3 x %Xn " °- 
and for n = 20 in 210 seconds 
dr = 0.042° C. 
or the heating produced was 2 X 10~ 4 ° C. per second. Whether, sup- 
posing A A' to be filled with water, a pyrheliometer may be constructed 
on this principle I have yet to learn. 
Other interesting results of the same kind might be mentioned. Thus 
if the screw stopcock, figure 2, is closed quickly (C being open) there is 
always a decided increment of pressure. In other words in consequence 
of the viscosity of air, the fine space at the plug is virtually a closure before 
the screw is checked by an actual closure. 
If the glass plates G are removed and the air space A is closed by a pipe , 
P, tapering to a neck as in figure 3, it becomes a closed organ pipe adapted 
for the measurement of acoustic pressure. It may be blown by an ad- 
justable embouchure,/, described heretofore {Science, 53, 1921 (47)), or 
