174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914, 
sions regarding the nature of these waves, and of the medium by which 
they are transmitted. . 
Prof. Nagaoka, of the University of Tokio, investigated the densi- 
ties, elasticities, moduli of rigidity, and the velocities of transmission 
of longitudinal and transverse waves, for the various kinds of rock that 
compose the earth’s crust. An important result is to show that while 
there is a slight increase of density in going from the Quaternary to the 
Archean rocks there is a large increase of elasticity, so that the rate of 
propagation of wave motion in the latter rocks is much greater than 
in the former, the velocities for longitudinal waves going as high as 6 
and 7 kilometers per second. It may be stated here that the velocity 
in an unlimited medium of steel would be 6.2 kilometers per second. 
Tt might then be inferred that at great depths in the earth the ratio of 
elasticity to density may continue to increase with depth to a certain 
point, so as to permit of the great velocities of wave transmission 
observable in earthquake phenomena. 3 
Prof. Milne concludes from the velocities of seismic waves at differ- 
ent. depths that the materials and general characters of the crust of 
the earth that are found at the surface may extend to a depth of about 
30 miles, but beyond that the material seems rapidly to merge into a 
fairly homogeneous nucleus. ‘This state probably extends to a depth 
of six-tenths of the radius, but the remaining four-tenths forms a core 
which differs in its physical, and possibly its chemical, constitution, 
from the outer portion. What the state of this nucleus is must be a 
matter largely of conjecture until we have a fuller knowledge of the 
state of matter when subjected to the vast pressures that must exist 
within the earth’s interior. 
Additional evidence that the earth as a whole is at least as rigid as 
steel is furnished by a study of tidal phenomena, and also by the varia- 
tion of latitude. That bodily tides are caused by the moon in the solid 
earth has been proved by Hecker; and Sir George Darwin found by a 
study-of the tides of long period at different ports, extending over an 
interval of 33 years, that the ocean tides are about two-thirds as great 
as if the earth were unyielding. He also showed that this is the ratio 
that theoretically should exist if the rigidity of the earth were that of 
steel. 
With regard to the variation of latitude, Euler showed that if the 
earth’s axis of rotation and figure do not coincide one axis will revolve 
about the other in a period of 305 days. All attempts to discover a 
periodicity in latitude variations, however, were futile until Chandler, 
by a study of a great mass of material, showed that a period exists, 
but instead of 305 days he showed a period of 427 days. It was then 
pointed out by Newcomb that Euler had assumed the earth to be 
perfectly rigid, and that by assuming its rigidity as equal to that of 
steel the period would become 457 days. The inference to be drawn 
from this evidence is that the earth is more rigid than steel, 
