72 EARTHQUAKES. 



a day, a week, or even several years. Strictly speaking, 

 they are a series of separate earthquakes, the resultant 

 vibrations of which more or less overlap. AVhenever a 

 large earthquake occurs it is generally succeeded by a 

 large number of smaller shocks. 



The seismic disturbance as regards time is, as Mallet 

 remarks, very often ' like an occasional cannonade during 

 a continuous but irregular rattle of musketry.' In the 

 New Zealand earthquake of 1848, shocks continued for 

 nearly five weeks, and during a large portion of the time 

 there were at least 1,000 shocks per day.^ 



The earthquake of Lisbon, which in five minutes 

 destroyed the whole town, was followed by a series of 

 disturbances lasting over several months. . After Basle 

 had, on October 18, 1356, been laid in ruins, it is stated 

 shocks followed each other for a period of a year. The 

 Calabrian earthquake was continued with considerable 

 strength for a year, and it is said that the earth did not 

 come completely to rest for ten years. During this can- 

 nonade the heavy shocks announced, as they do in most 

 earthquake countries at the present day, a series of weaker 

 disturbances. In certkin exceptional cases this order of 

 events has been inverted, and slight shocks have an- 

 nounced the coming of heavy ones. Fuchs gives an ex* 

 ample of this in the earthquake of Broussa, when the first 

 shock was on February 28, 1855. On March 9 and 23 

 there were heavier shocks, but the heaviest did not arrive 

 until March 28. 



Under certain conditions it is possible to have a 

 sensible vibration produced in the ground which is 

 practically of unlimited duration ; thus, for instance, it 

 has been noticed that the falling of water at certain large 

 waterfalls, by its continuous rhythmical impact on the 

 » West Rev., July 1849. 



