PHYSICS: C. BARUS 
fauna and flora at the bottom. The details of this work, including 
maps, graphs and tables, are to be published by the Carnegie Institu- 
tion of Washington. The relation of local conditions to the precipita- 
tion of CaCOs, thus decreasing the depth of the water, is pointed out. 
Studies of the effect of these changes on organisms were made. The 
limiting factor for plants seems to be fixed nitrogen. Only 0.02 mgm. 
of fixed nitrogen per liter could be determined and it was not thought 
practicable to determine local changes with certainty. The limiting 
factor for animals seems to be food. Oxygen could easily become a 
limiting factor. One kilogram of fish would use up all of the oxygen 
in 4300 liters of water of the lowest 02-concentration found at the sur- 
face, in twenty-four hours. It seems improbable that fish alone would 
suffocate, but swarms of Dinofiagellates might suffocate themselves and 
other animals present. 
NOTE ON INTERFEROMETER METHODS OF MEASURING THE 
ELASTICS OF SMALL BODIES 
By Carl Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITY 
Communicated, October 26, 1917 
1 . Method. — At the request of Prof. W. G. Cady, who was in need of 
Young's modulus in case of certain crystals used in experiments in 
which he is interested, I endeavored to adapt for this purpose the inter- 
ferometer heretofore^ described for measuring small angles with an 
auxiliary mirror. The project seems feasible and apparently simple in 
execution, when the method of end thrust indicated in figure 1, is used. 
Here F is a rigid metallic bar subjected to a force couple, carrying the 
coplanar mirrors m, m' ^ and capable of rotating slightly in a horizontal 
plane. These mirrors receive the corresponding rays, a, of the inter- 
ferometer. The force couple is resisted by the resilience of the rods, 
r, r' ^ to be tested, as these push respectively against the ends of the 
bar, F, and against the rigid abutments, A\ of the apparatus. If 
the force couple changes the bar, F, rotates correspondingly. The 
component rays, a, b, then register the amount of rotation in the 
interferometer. 
To apply the force couple, weights suspended from the stationary 
pulleys, c, c\ were utilized. These actuate the rectangular offsets, s, 
s\ which force their conical ends, e, e' ^ into corresponding depressions of 
the bar, F. When not under stress, F is supported by the double 
