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, 



