192 EARTHQUAKES. 
which the disturbance emanated; in the other—or 
transverse—set, the direction of motion was at right 
angles to that line.” 
It is also found that a hill does not cut off the vibra- 
tions to any large extent, although their direction is 
mostly transverse as they ascend its side; a pond of 
water, on the other hand, completely cuts off the vibra- 
tions, which creep gradually around its margin. 
The transverse vibrations continue fora longer time. 
From the report of General Abbot relating to the ex- 
plosion at Hallet’s point, and the observations of Mr. 
Mallet in his experiments at Holyhead, we learn that 
‘the transit velocity increases with an increase in the 
intensity of the initial shock.’’ And a higher velocity 
is attained when an explosion occurs under deep, than 
under shallow water. 
General Abbot found that ‘ta high magnifying power 
of a telescope is essential in seismometric observations ”’ ; 
that the velocity of transmission is higher in proportion 
to the violence of the initial shock, and ‘‘ diminishes as 
the general wave advances”; that ‘‘ the movements of 
the earth’s crust are complex, consisting of many short 
waves first, increasing and then decreasing in ampli- 
tude ; and with a detonating explosive, the interval be- 
tween the first wave and the maximum wave is shorter 
than with a slow burning explosive.” 
Observations taken at the time of earthquakes indi- 
cate that motion may vary in its direction, and that the — 
amplitude of the earth’s movements rarely exceeds 
three or four millimeters; and when it is beyond the 
elastic limits of the material, fissures in the earth are 
formed which often open and close with the vibratory 
movements. 
The condition and nature of the underlying soil ina 
measure determines the intensity of the shock, or may 
neutralize it entirely. The wave, in passing from one 
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