EARTHQ UAKE-PHENOMENA. 



521 



1800 to 1850, or one in about five days. The means of detecting and 

 recording shocks are now so perfected, that, when applied in all parts 

 of the globe, they will, doubtless, fully justify our statement that in no 

 instantof time is the earth's crust free from vibrations. The seismo- 

 graph is an instrument for the " automatic registration of earthquake- 

 shocks." 



An interesting account of this instrument, by George Forbes, 

 was published in the September number of The Popular Science 



Monthly. 



Earthquakes have been defined to be a " travelling zone of vibra- 

 tion." The movement is in every direction from the area of disturb- 

 ance, and the velocity depends on the substance and structure of the 

 material through which it is transmitted. In New Zealand, in 1848, 

 people on the shore witnessed the disastrous progress of the earthquake 

 along the mountains before they felt the shocks. At Messina, during the 

 Calabrian earthquake, the terrified inhabitants saw villas overthrown 

 upon the coast by shocks which they had not felt, but which in a mo- 

 ment laid in ruins a portion of their own city. The velocity with 



Fig. 8. 



View of the Fokt of Sindree feom the West, in Maeoh, 1838. 



which the vibrations travel has been a subject of careful investigation. 

 The Lisbon earthquake moved about 20 miles in a minute ; that which 

 occurred in 1819, in the delta of the Indus, appears to have moved at 

 the rate of 53 miles in a minute, or nearly 5,000 feet in a second. 

 Other observations show that the movement may be from 1,000 to 

 5,000 feet per second. It has been ascertained that in blasting rocks 

 the vibrations move in a second from 1,000 to 1,700 feet. The sound- 

 waves move more rapidly, and, for this reason, shocks are usually pre- 

 ceded by subterranean rumbling. The velocity of sound through 

 uniform strata is ascertained to be from 8,000 to 10,000 feet in a sec- 

 ond. Tyndall found that sound-waves moved through burnt clay 

 nearly ten times more rapidly than through air at a temperature of 

 32 Fahr. From this the phenomena of earthquake movement might 



