Present Problems of Geophysics. — Becker. 1 7 
under purely cubical compression dense, undeformed solids 
are perfectly elastic. Hence the basal problem of geophysics 
is to find the law of elastic compressibility. This can not be 
accomplished by direct means, but the task is, nevertheless, 
as pointed out above, not a hopeless one, and has been taken 
in hand. Should success be achieved, researches will follow 
on the variation of elasticity with temperature. This 
feature of the investigation will present very great experi- 
mental and theoretical difficulties, but there is no good 
reason to despair of success. 
When the law of resistance of solid bodies becomes 
known as a function of both temperature and pressure, even 
for isotropic substances with only two moduluses of elastic- 
ity, the way will be opened to various important investiga- 
tions, largely mathematical in character. It is true that 
thoroughly isotropic bodies are seldom met with, yet geo- 
logical masses must, nevertheless, often approach closely to 
this ideal. Many of the most important rocks are chiefly 
composed of triclinic feldspars, which, indeed, occur about as 
abundantly as all other minerals found at the surface of the 
earth put together. A triclinic feldspar crystal rejoices in 
the full possible number of elastic moduluses, 21. Yet a 
large spherical mass of small, fortuitously oriented feldspars 
will behave to external forces of given intensity and direction 
in the same way, no matter how the sphere may be turned 
about its center, and will, therefore, act as an isotropic body. 
This fact is enough to show that an indefinite variety of inti- 
mate molecular structures are compatible with molar iso- 
tropy. 
Thus a knowledge of isotropic elasticity will suffice as a 
basis for testing reasonable hypotheses of the constitution of 
the earth's interior, taking into account its known rigidity 
and density. Still greater light can be thrown on this subject 
by including in the investigation the moon and Mars ; for 
their masses and dimensions are known and there seems 
every probability that they are composed of the same mate- 
rials as the earth, though in different proportions. If a given 
hypothesis as to the chief constituents satisfies the known 
conditions of all three planets, it will doubtless find accept- 
ance. Such a result would open the way to fresh advances 
