208 
PHYSICS: C. BARUS 
Proc. N. A. S. 
but it is nevertheless definite. When a large resistance, r, put in the 
telephone circuit produces a fringe deflection, s, the equation sr = con- 
stant is near enough for practical purposes and thus the passage from s to 
p (when called for) is very convenient. 
It is the object of the present paper to continue these experiments by- 
treating pressure distributions in vibrating regions of different size and in 
slender pipes. The case of wide organ pipes I have also investigated, but 
the results must be given later. 
In figure 1, the two shanks of the mercury U-tube at least 6 cm. in diam- 
eter (R, air volume 48 cm. 3 , R f , air volume 135 cm. 3 ), are closed on top 
by glass plates, to receive the rays of the vertical interferometer; i.e., 
the component rays enter R and R' ', normally to the diagram. 
U-tube Used Differentially. — The two different reservoirs R and R f of the 
U-tube, figure 1, were placed in communication by the branch pipe b, b' f 
containing the three way cocks, D, E, shutting off the air vents a, d, and 
leading to the telephone T through the quill tubes t and t'. The pinhole is 
in the second branch t" at O and a cock, C (here open), is interposed. I 
had anticipated a differential fringe deflection, owing to the different 
resonance volumes at R and R' '. The result, however, was absolutely 
negative. Each reservoir (R, R') thus acts like an open air communi- 
cation with reference to the other, so that the effect of the pinhole valve 0 
vanishes. On using either side separately (t b R closed, R' D a open to the 
atmosphere, or t b' R' closed, R E d open) the normal behavior at once 
appeared. 
The only way of securing a differential effect detected, was by elongating 
either branch (6, for instance, by inserting a long piece, 40 to 80 cm., of 
rubber tubing) . No doubt, this is merely equivalent to stopping, partially, 
access to either chamber, R or R'. 
A variety of correlative experiments were made with the simple (non- 
differential) apparatus (t b R closed R' D a open). I have elsewhere re- 
ferred to the absence of fringe displacement (nearly) when the copper foil 
carrying the pinhole is cemented to the mouth of a funnel tube; but the 
prolongation of the quill tube t", if the diameter is not increased, is almost 
indefinitely permissible. 
The experiments made suggest a method of obtaining an effect which is 
at least apparently differential. For this purpose the cock C (fig. 1) is 
to be closed, so that c is inactive, and the pinhole c (fig. 1) to be inserted 
either into the branch b' or b. With a normal fringe displacement of 40, 
the pinhole in b' gave a displacement of 38 fringes; in b correspondingly 27 
fringes. Hence the large volume R r is more favorable to a larger displace- 
ment than the smaller volume R. It will be observed at once that here 
these reservoirs act merely like the outside air in the case of the original 
experiment. Thus there is no true differential effect resulting from the 
