PHYSICS: C. BARUS 
49 
6. Conclusion. — The present experiments made with a totally dissimilar 
apparatus and in a different manner, are nevertheless (notwithstanding the 
relative simphcity of the present design) not markedly superior to the earlier 
experiments (1. c), as a whole. The misgiving which I felt (see § 1) regard- 
ing the force couples entering into the earlier method was not therefore justi- 
fied. Both apparatus function admirably so far as the optics of the method 
are concerned. This is particularly noteworthy when one considers the 
admissibihty of the rather rough treatment needed in work of the present 
kind. Both apparatus are liable to give misleading results from the same 
cause; i.e., from an insufiiciently uniform and continuous contact of the two 
ends of the rod with the abutments. From this results appreciably unequal 
distribution of stress in the sections of the rod and possibly flexure. There 
seems to have been no serious yield in the abutments, etc., of either apparatus. 
The values of the modules £ as a consequence come out too small. There 
can therefore (tapping admitted) have been no serious discrepancy from fric- 
tion in the application of stress; for this would have made E too large. More- 
over all slight dislocations within the interferometer as the result of any rea- 
sonable jar were finally eliminated, so that the cycles practically closed or 
merely gave evidence of a difference of slope in the outgoing and return series. 
Such an effect would be expected from viscosity and hysteresis. 
I was at first inclined to regard the small values of the modulus E as an 
actual or trustworthy result, in keeping with the peculiar crushing stress 
applied. But inasmuch as E may be increased to the normal value by succes- 
sively decreasing the diameter of the rod, in the case of glass and even of 
brass, the small values of E must be associated with the lack of contact at the 
abutments of the rod. Rods about 1 to 2 cm. in length should not be thicker 
than 1 or 2 millimeters (ratio about 10 to 1), if results are to be assured in 
their absolute values. And here again a thin rod, r, with two thick ends, if 
both ends are firmly clutched without strain, is the ultimate desideratum. 
Figures 3, 4, 2 (sheath, s), are admissible expedients, the latter being partic- 
ularly convenient. The relative results are almost always smooth and admi- 
rable to a fraction of a wave length; but for relatively large sections they can 
not be interpreted owing to the sectional discrepancy in question. This also 
' is relative in its character; at least for moduh markedly above 10^°. Thus it is 
as difficult to obtain the true modulus for a glass rod as for a brass rod, 
although the latter body is far more rigid. 
It is not easy to interpret the apparant hysteresis in many of the above 
graphs; for this is always associated with possible changes in a complicated 
train of apparatus. Similarly the different rates in the outgoing and the 
return series may be variously explained. If the measurements are made in 
triplets between definite steps of pressure, this difference soon vanishes. 
Hence this procedure is to be preferred. 
1 Abridged from a forthcoming report to the Carnegie Institution cf Washington, D. C. 
2 These Proceedings, 3, 1917, (693-696). 
