156 THE POPULAR SCIENCE MONTHLY 
one of America’s prominent seismologists, and his testimony may be 
regarded as specially competent. The three shocks he noted may have 
been three separate impulses coming from the three epicenters pre- 
viously mentioned, at Dubuque, Waukegan and Bloomington, started 
by slippings closely following each other in each of these places. 
This inference is in a measure strengthened by some observations 
made on the duration of the earthquake. ‘There are in all fifty-eight 
such observations, showing a range of estimates from one second to 
three minutes. Thirty-eight of these estimates vary from one to eight 
seconds and average four seconds. In six places the disturbance is 
reported to have lasted ten seconds; in five places, fifteen seconds, and 
in one, twenty. An average of these twelve estimates is about thirteen 
seconds. In two places the shock is reported as lasting a half minute; 
in three places, one minute; in one, a minute and a half, while in Dixon 
and Joliet the disturbance continued for three minutes. No great 
accuracy can be claimed for these estimates, but it will be observed that 
they fall into three groups, one with an average of four seconds, one 
with an average of about thirteen seconds and another with an average 
of about sixty seconds. We may suppose that the shortest average 
represents places where only one of the three shocks was sensible, while 
the two larger averages represent places where two or where all three 
shocks were strong enough to be felt. All places where the disturbance 
lasted more than a minute are somewhat centrally located, and may 
hence very well have been exposed to the effects of all the three shocks, 
each of which increased the total length of the period of the disturbance. 
No less than sixty-six observations are reported on the time at which 
the earthquake was felt. These are of interest chiefly in showing how 
great is the difference in accuracy of time measurements required for 
general purposes, and for the purpose of seismic investigations. They 
also illustrate our general preference for round numbers. The reports 
range from eight o’clock in the morning to twenty minutes after nine. 
More than half of them give the numbers thirty, thirty-five, forty and 
forty-five minutes after eight. Discarding these figures, which are 
multiples of five, twenty-two observations range from thirty-seven to 
forty-one minutes after eight. The time recorded by the seismometer 
in the office of the United States Weather Bureau in Peoria, no doubt 
more reliable, was thirty-eight minutes after eight. The time marked 
by another government seismometer in Washington was forty-one and 
a half minutes after eight. If the velocity of the earthquake wave in 
traveling from Peoria to Washington, be calculated from these last two 
figures, we find that it approached three and three tenths miles per 
minute. For the purpose of determining the velocity of earthquake 
waves the data furnished by the press reports are of course entirely 
inadequate. 
The location of the epicentral tracts and of the mesoseismal area, is, 
