EARTHQUAKES AND OTHER SEISMIC MOVEMENTS. 309 



during an earthquake the observer's feelings distinctly tell him that 

 there are several maxima. The chief results which investigators have 

 aimed at have been the measurement of the amplitude, period, direc- 

 tion, and duration of the motions ; and attention has been given to the 

 velocity with which the disturbance is propagated. 



If we were to ask the inhabitants of a town which had been shaken 

 by an earthquake the direction of the motion they had experienced, it 

 is not unlikely that their replies would include all the points of the 

 compass. Many, in consequence of their alarm, have not been able to 

 make accurate observations. Others have been deceived by the mo- 

 tion of the building in which they were situated. Some tell us that the 

 motion was north and south, while others say that it was east and west. 

 A certain number have recognized several motions, and among the rest 

 there will be a few who have felt a wriggling or twisting. Leaving 

 out exceptional cases, the general result obtained from personal obser- 

 vation as to the direction of an earthquake of moderate intensity is 

 extremely indefinite, and the only satisfactory information to be got is 

 that derived from instruments or from the effects of the earthquake as 

 exhibited in shattered buildings and bodies which had been overturned 

 or projected. By the use of seismographs it has been shown that dur- 

 ing an earthquake the ground may move in one, two, or several direc- 

 tions, and it is only when a decided shock is exj>erienced that we can 

 determine with any confidence the direction in which the motion has 

 been propagated. The apparently twisting or wriggling motions are 

 supposed to be the result of combinations of linear movements in 

 different directions. It is often difficult, when reading accounts of 

 earthquakes, to determine the length of time a shaking was continu- 

 ous. Disturbances which succeed one another with sufficient rapidity 

 to cause an almost continual trembling of the ground may be regarded 

 as collectively forming one great seismic effort, which may last a min- 

 ute, an hour, a day, a week, or even several years. Strictly speaking, 

 they are a series of separate earthquakes, the vibrations of which more 

 or less overlap. Whenever a large earthquake occurs, it is generally 

 succeeded by a considerable number of smaller shocks. Disturbances 

 of this character are compared by Mallet to " an occasional cannonade 

 during a continuous but irregular rattle of musketry." Continuous 

 motions perceptible to our senses without the aid of instruments usual- 

 ly last from thirty seconds to about two or three minutes. The princi- 

 pal vibrations or shocks of the disturbance occur at unequal intervals ; 

 and in the periods of vibration there are irregularities in any given 

 earthquake, and different earthquakes differ from one another. The 

 extent of the movement is much less than the feelings of one experi- 

 encing a shock would lead him to estimate it. It is usually within the 

 fraction of an inch in either direction. According to Dr. Wagener, 

 the earth's horizontal motion at the time of a small earthquake is usu- 

 ally only the fraction of a millimetre, and it seldom exceeds three or 



