ON THE EARTHQUAKE PHENOMENA OF JAPAN. 213 
yielding. The effect is that of a sudden push. The area thus affected 
my colleague, Professor, T. Alexander, has called the core of the earth- 
quake. The existence of an earthquake-core is one means of explaining 
the enormously high velocities of propagation which I and other observers 
have from time to time recorded. After the push or shock come the re- 
sulting irregular tremors. These continually slow down in their period 
until, when they reach a period of two or three seconds, the seismograph 
ceases to act. The slow irregular surging of a level appears to be a con- 
tinuation of the record of a seismograph. 
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In these respects the vibratory motions of an earthquake are analogous 
to a spectrum of light—there being two extremities which with ordinary 
instruments are usually unobserved—at one end because the vibrations 
are too quick, and at the other end because they are too slow. The 
accompanying diagram of the earthquake of March 11, 1882, shows the 
portion of an earthquake registered by an ordinary seismograph. 
A set of experiments which I am now engaged upon in Japan has for 
its object the determination of some true measure of the intensity of an 
artificial disturbance produced by the explosion of a charge of dynamite 
as it radiates from its origin. Rather than estimate the intensity of an 
impulse at a point, by vague terms or by an arbitrary scale of degrees, I 
have attempted to measure the intensity of a shock by the stresses it is 
