EAKTHQUAKE MOTION DEDUCED FEOM OBSERVATION. 71 



of rectilinear motion, as, for instance, the normal and 

 transverse movements, maj have led observers to imagine 

 that tiiere has been a twisting motion taking place, and 

 this especially when the two sets of movements have 

 quickly succeeded each other. 



Persons inside flexible buildings may possibly have 

 experienced more or less of a rotatory motion, although 

 the shock was rectilinear ; the building assuming, such a 

 motion in consequence of its construction and its position 

 with regard to the direction of the shock. 



In the case ai destructive earthquakes, especially at 

 points situated practically above the origin, the universal 

 testimony. Mallet tells us, is that a twisting, wriggling 

 motion in different planes, attended by an up-and-down 

 movement of greater range, is experienced. To such dis- 

 turbances the word sussultatore is sometimes applied. 

 Mallet has given many elliptical and other closed curves 

 to illustrate the nature of such motions. 



Duration of an Earthquake, — ^When reading accounts 

 of earthquakes it is often difficult to determine the length 

 of time a shaking was continuous. In Japan, in a.d. 745, 

 there was a shaking which is said to have lasted sixty 

 hours; and in a.d. 977 there were a series of shakings 

 lasting 300 days. Often we meet with records of disturb- 

 ances which have lasted from twenty to seventy days. 



At San Salvador, in 1879, more than 600 shocks were 

 felt within ten days ; in 1850, at Honduras, there were 

 108 shocks in a week ; in 1746, at Lima, 200 shocks were 

 felt in twenty-four hours ; at the island of St. Thomas, in 

 1868, 283 shocks were felt during about ten hours. 



Disturbances like these, which succeed each other with 

 sufficient rapidity to cause an almost continual trembling 

 in the ground, may be regarded as collectively forming 

 one great seismic effort which may last a minute, an hour, 



