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THE POPULAR SCIENCE MONTHLY. 



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noteworthy fact that there is in general no one motion standing out 

 from the rest as greatly larger than those which precede and follow it. 

 The direction of motion varies irregularly during the disturbance so 

 much so, that in a protracted shock the horizontal movements at a sin- 

 gle station occur in all possible azimuths " (that is to say, to all points 

 of the compass). " The duration, that is to say the time, during which 

 the shaking lasts at any one point is rarely less than one minute, often 

 two or three, and in one case in the writer's experience was as much as 

 twelve minutes." 



The horizontal path pursued, in an actual earthquake at Tokio, on 

 March 8, 1881, by the part of the recording instrument which was fixed 



to the ground, is shown in the an- 

 nexed figure.* It is magnified 

 six fold, and the time occupied 

 from the beginning to the end of 

 this part of the vibration was 

 three seconds. This earthquake, 

 although alarming, did no dam- 

 age except to crack a few walls. 



It is obvious that when the 

 motion is so complicated, the im- 

 pressions of people present go for 

 little as compared with an auto- 

 matic record. Observers often differ widely among themselves as to 

 what was the direction of the prevailing oscillation, and the magnitude 

 of the displacement of the ground is generally much exaggerated. It is 

 true that in some of the great historic earthquakes the displacements 

 are supposed to have been considerable ; for example, according to 

 Mallet, in the Neapolitan shock of 1857 it amounted to a foot, and 

 Abella assigns six feet as the amplitude in the Manila earthquake of 

 1881. But, without contesting the accuracy of these estimates, it is 

 safe to say that such displacements are very rare, for, as proved by 

 automatic seismographs, when the motion is as much as a quarter of 

 an inch, brick and 6tone chimneys are genei-ally shattered. 



Every railway-traveler knows that it is not the steady speed, but 

 the starting and stopping, which jars him ; that is to say, it is change 

 of velocity by which he is shaken. The misconception of an observer 

 in an earthquake arises from the fact that the sensation of being tossed 

 about comes from the change of velocity to which he is subjected, 

 rather than from the extent of his displacement. Now, the greatest 

 change per second of velocity may be considerable in a vibration, 

 while the amplitude is small. 



The force of gravity is the most familiar example of a change per 

 second of velocity, for in each second the velocity of a falling body is 

 augmented by a velocity of thirty-two feet a second. Ewing appears 

 f " Memoirs of the Science Department of the University of Tokio," No. 9, 1883, p. 58. 



