8 22 THE POPULAR SCIENCE MONTHLY. 



once at the bottom of the silver-mines of Charnacillo, more than six 

 hundred feet underground, during an earthquake that destroyed his 

 own house and several others right above him, while he did not feel 

 the least agitation. 



The duration of the shocks is generally very brief, sometimes not 

 more than a second or two. The undulatory movements are more pro- 

 longed. A few instants are sufficient to produce the most disastrous 

 effects. Three shocks, each of which was estimated to be not more 

 than four seconds long, destroyed more than 20,000 persons in and 

 around Caracas in March, 1812 ; and the convulsion at Pdobamba, in 

 1797, killed as suddenly 30,000 victims. 



But, however overwhelming and disastrous in reference to our per- 

 sons and buildings earthquakes may be, it must be borne in mind that 

 the amplitude of their movements is wholly insignificant in propor- 

 tion to the dimensions of the globe whose epidermis they shake. The 

 phenomenon is rarely limited to a single shock. Generally, several 

 shocks follow one upon another at short intervals. In many cases, 

 the jnovements are repeated for months and years, with pauses of a 

 variable duration, so as to form as a whole, till they are totally extin- 

 guished, what might be called a seismic period. After the shock which 

 overthrew Thebes, on the 18th of August, 1851, the commotions con- 

 tinued in Boeotia for eleven months, occurring sometimes as often as 

 three times in twenty-four hours. Long series of shocks disturbed a 

 part of Scotland during the two years from the 20th of October, 1839, 

 to the 7th of December, 1841. Hundreds of similar examples might 

 be cited. 



The chain of the Alps has furnished examples of seismic periods in 

 many of its parts. Series of shocks were felt at Pignerol, in Pied- 

 mont, from the 2d of April, 1808, till the 17th of May following, dur- 

 ing which time not a day passed but some movement was felt. Some- 

 times the tremors were noiseless, sometimes they were accompanied 

 with commotions preceding the destruction of buildings. The com- 

 motions were renewed on the 26th of September, the 28th of October, 

 and the 22d of November ; and in the next year on the 13th of March 

 and 2Gth of June. Similar periods of seismic action were observed in 

 le Valais in 1755, on Lake Gardo in 1866, on Monte Baldo in 1868, 

 and at Belluno in 1873. The present period in Andalusia is of the 

 same kind. The prelude of the 23d of December, which disturbed a 

 part of the Spanish Peninsula, the great earthquake of the 25th of De- 

 cember, and that long succession of shocks which still continued on 

 the 9th of March with sufficient violence to cause new ruins, belong to 

 the same series. As Humboldt has remarked, it is noteworthy that 

 series of this kind are produced more especially in countries distant 

 from volcanoes. 



In the disturbed regions we may generally remark a tract of lim- 

 ited extent in which the movement is particularly energetic. It cor- 



