66 
PHYSICS: C. BARUS 
Proc. N. a. vS. 
if but a single square cm. on each glass face radiates toward m. As there 
are a number of such square cms., contributing radiation, the coefficient 
of (3) would be decreased ten or twenty times. One may conclude I 
think that compared with i^jy is numerically much in excess. This 
implies too large a A^, to persist in a room of practically constant temper- 
ature, in the summer. It is thus more than probable that at a vacuum of 
10"^ mm. or a force of .22 dynes on each shot, so small a part as 10 or 20 
times the 10 ~^ X 70 X instanced in equation (1) may remain fluctuating 
in value with small changes in the radiating thermal environment. 
Measurement of 7 in Terms of the Viscosity of Air. — A maximum deflection 
of the w-balls is produced by placing the M-balls in position. When this 
is attained, the latter are reversed and the velocity v with which the 
needle (filamentary frame) passes through the equilibrium position, is 
measured with a stopwatch. The M-balls are then again reversed and the 
measurement of v repeated; etc. In a plenum, the motion of the needle 
through the zero point js practically uniform and therefore the frictional 
force is equal to the gravitational pull. Moreover, if two points of the 
scale near to and equidistant from the zero point are selected for releasing 
and arresting the stopwatch, the torsion of the fibre acts to the same de- 
gree as an acceleration and a retardation. 
The frictional resistance encountered by either round mass m of radius 
r is by Stokes' equation ^ir'qrv, where 77 is the viscosity of air. The ve- 
locity V of the balls w is , 
V = {l/2L)^y/ M 
Ly is the telescopic excursion in the time A/, symmetrically, to the posi- 
tion of equilibrium. We obtain in this way 
B}_ STrrjrl Ay 
^ Mm L At 
Work on this principle has been actively pursued, but the summer 
closed before it could be completed. 
* Advance note from a Report to the Carnegie Institution of Washington, D. C. 
PLANE REFLECTION OF SOUND, AS EXHIBITED BY THE PIN- 
HOLE RESONATOR'' 
By C. Barus 
Department of Physics, Brown University 
Communicated February 24, 1922 
1. Vertical Reflectors. — These experiments, made between the walls of a 
room (y direction), with the closed organ pipe P, vertical, mouth down- 
