l6 ABOUT VOLCANOS AND EARTHQUAKES. 



substance on which it is delivered with the rapidity of 

 sound in that substance. Perhaps it may be new to many 

 who hear me to be told that sound is conveyed by water, 

 by stone, by iron, and indeed by everything, and at a dif- 

 ferent rate for each. In air it travels at the rate of about 

 1140 feet per second, or about 13 miles in a minute. In 

 water much faster, more than four times as fast (4700 

 feet). In iron ten times as fast (11,400 feet), or about 

 130 miles in a minute, so that a blow delivered endways 

 at one end of an iron rod, 130 miles long, would only 

 reach the other after the lapse of a minute, and a 

 pull at one end of an iron wire of that length, would 

 require a minute before it would be felt at the other. 

 But the substance of the earth through which the shock 

 is conveyed is not only far less elastic than iron, but it 

 does not form a coherent, connected body ; it is full of 

 interruptions, cracks, loose materials, and all these tend 

 to deaden and retard the shock : and putting together all 

 the accounts of all the earthquakes that have been ex- 

 actly observed, their rate of travel may be taken to vary 

 from as low as 12 or 13 miles a minute to 70 or 80 : 

 but perhaps the low velocities arise from oblique waves. 

 (23.) The way, then, that we may conceive an earth- 

 quake to travel is this, I shall take the case which is 

 most common, when the motion of the ground to-and- 

 fro is horizontal. How far each particular spot on the 

 surface of the ground is actually pushed from its place 

 there is no way of ascertaining, since all the surrounding 

 objects receive the same impulse almost at the same in- 

 stant of time, but there are many indications that it is 



