WILLIAM G. STEVENSON. 191 
In obtaining a clear idea, therefore, of vibratory mo- 
tion, and in proving that earthquake motion is of this 
character—the foundation was Jaid for the more com- 
plete unfolding of the idea advanced by Young—which 
Robert Mallet, in 1846, formulated into a definition which 
described an earthquake to be—‘‘ the transit of a wave 
or waves of elastic compression in any direction from 
vertically upwards to horizontally, in any azimuth, 
through the crust and surface of the earth, from any 
center of impulse or from more than one, and which 
may be attended with sound and tidal waves, dependent 
upon the impulse and upon circumstances of position as 
to sea and land.”’ 
To the waves of elastic compression and extension— 
which may be designated the rectilinear motions of the 
molecules—Prof. Milne adds, what he calls the waves 
of elastic distortion—which may be regarded as the 
twisting or contorting motion of the masses—as essential 
to a complete definition of earthquake motion. 
These elastic waves doubtless exist—but they are seen 
only by the ‘‘ mind’s eye,”’ and are known by processes 
of reasoning and not by direct experimental evidence. 
‘CA true surface undulation,” like a water-wave, is the 
only kind of wave motion which thus far responds to 
the tests applied. 
Seismology—or that department of physical science 
which relates to the study of earthquakes—has, largely 
through the labors of Mr. Mallet, made much progress 
during the last forty years towards solving the compli- 
cated questions pertaining to earthquake phenomena. 
In the experiments which have been made by Milne 
and Gray to determine the effects produced in the earth’s 
crust by falling weights—it is found that two distinct 
sets of vibrations are caused. ‘‘In one set—the normal,”’ 
says Milne,—‘‘the direction of motion was along a line 
joining the point of observation with the point from 
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