FORM AND CONSTITUTION OF EARTH—STEWART. 173 
been situated at a greater depth than 30 miles below the earth’s sur- 
face, and probably not below 20 miles, from which it would appear 
that any transfer of material that may occur at greater depths must 
take place without shock, and that consequently the material there 
must behave as a fluid. 
The most valuable evidence regarding the earth’s interior is 
afforded by the study of earthquake phenomena. An earthquake 
shock occurs in some part of the world, and at once elastic vibrations 
are set up in the surrounding material which are propagated in all 
directions from the center of disturbance, and leave their records upon 
the seismographs installed at stations in various parts of the world. 
By a study of these records some important conclusions can be drawn. 
In the first place it is found that the earthquake waves that reach 
stations within about 20° of the center of disturbance are of an 
entirely different character from those observed at more distant sta- 
tions. ‘The former are confined to the earth’s crust, while the latter 
travel through the earth by the shortest route, or possibly by brachy- 
stochronic routes, or routes of shortest time. It is with the latter that 
we are chiefly concerned. 
A study of a great mass of data regarding these long distance waves 
has revealed the following facts: 
These waves may be divided into preliminary tremors—of which 
there are two phases—and large waves. The time required for the 
waves of the first phase to travel from an earthquake center to a 
distant station is proportional to the length of the chord drawn 
between those points. From this it may be inferred that these waves 
travel at a uniform rate along chords, that rate being about 9.25 
kilometers per second. The times required for waves of the second 
phase to reach the distant stations are not proportional to the lengths 
of the chords, but show a velocity increasing with the distance trav- 
eled according to some law not yet understood. The large waves are 
propagated along the surface of the earth, and have a uniform velocity 
of about 2.95 kilometers per second. 
The generally accepted view regarding the preliminary tremors is 
that those of the first phase are longitudinal waves, while those of the 
second phase are transverse or distortional waves. As the velocity 
of propagation of a longitudinal wave varies as the square root of the 
ratio of the volume-elasticity to the density of the medium, it appears 
that, whatever the composition of the interior of the earth, the ratio 
of volume-elasticity to density must be constant at all depths, and 
therefore constant with varying density of the medium traversed. 
This is a property of gases. If the waves of the second phase are trans- 
verse waves it would appear that this same medium is capable of trans- 
mitting such waves, which can not be transmitted by fluids or gases. 
Until the law of their increase of velocity with distance, and therefore 
with depth, is better understood, it is too soon to draw any conclu- 
