MAR. 4, 1923 PROCEEDINGS: PHILOSOPHICAL SOCIETY 93 
Dr. L. J. Briaas presented an informal communication on Direct measure- 
ment of air speed in wind-tunnels. 
Author’s Abstract: At the request of the National Advisory Committee 
for Aeronautics, the Aerodynamical Physics Section of the Bureau of Stand- 
ards has made some direct determinations of the air speed in a wind-tunnel 
for comparison with pitot-tube measurements of air speed. 
The method employed was as follows: 
A small parallel pencil of light from an are source was so reflected that 
it traversed the tunnel three times at right-angles to the longitudinal axis 
of the tunnel and was finally focused upon a photographic film carried on the 
surface of a rapidly rotating drum. The distance between adjacent beams 
measured along the axis of the tunnel was approximately one meter. 
Small balloons filled with hydrogen and weighted so as to have the same 
average density as the air-stream were carried through the tunnel with the 
air-stream. Each balloon eclipsed successively the three light-beams and 
the instant of the eclipse was indicated by a break in the trace on the photo- 
graphic film. Timing lines, representing time intervals of one-thousandth 
of a second, were simultaneously recorded on the film with the aid of a 500 
cycle tuning-fork operated by an electron-tube drive. By this means the 
time required for the balloon to travel from one light-beam to the next 
could be measured, which, in connection with the known distance between 
the light-beams, gave the data necessary to determine the air-speed. The 
pitot-tube pressure developed by the air-stream was also continuously 
recorded on the film with the aid of a diaphragm gauge equipped with a 
mirror. Preliminary measurements indicate that the mean of the speed 
determinations by the balloon method agrees with the air-speed measure- 
ment as determined by the standard pitot-tube to within two-tenths of one 
per cent. 
Adjournment at 9:55 P.M. was followed by a social hour. 
875TH MEETING 
The 875th meeting was a joint meeting with the Washington Academy of 
Sciences in the Cosmos Club Auditorium on Thursday, December 21, 1922. 
The meeting was addressed by Dr. H. A. Cuark of the Taylor Instrument 
Company of Rochester, New York, on The manufacture of thermometers. 
The address was illustrated by lantern slides, and some specimens of ther- 
mometer tubing and of thermometers were exhibited. The paper was dis- 
cussed by Messrs. HumpHreys, JONES, Kapret, and Hawxswortu. The 
meeting adjourned at 9:40 P.M. 
876TH MEETING 
The 876th meeting was held in the Cosmos Club Auditorium January 13, 
1923. It was called to order at 8:30 P.M. by President Wuire, and 62 per- 
sons were in attendance. 
The address of the evening was given by the retiring President, E. C. 
CRITTENDEN, on The measurement of light. The address was discussed 
by Messrs. Pawuine, L. H. ApAMs, HAWKSWoRTH, WILLIAMSON, and Mouuer. 
It will be published in an early number of the Journal of the Washington 
Academy of Sciences. 
Adjournment at 10 P.M. was followed by a social hour. 
J. P. Auut, Recording Secretary. 
