224 ANNUAL OF SCIENTIFIC DISCOVERT. 



crust of lava, through which it had broken, as to see its walls for quite 

 150 feet down by estimation. They were glowing hot to the very 

 lips, although constantly evolving a torrent of rushing steam with vary- 

 ing velocity. Accustomed as I have been, by profession, for years to 

 judge of temperatures in large furnaces by the eye, I estimated the 

 temperature of this mouth, by the appearance of its heated walls, at 

 the lowest visible depths ; they were there of a pretty bright red, visi- 

 ble in bright winter sunlight overhead. I have no doubt, then, that the 

 temperature of the shaft at from 300 to 500 feet down was sufficient to 

 melt copper, or from 1900 to 2000 of Fahrenheit. From the ex- 

 tremely bad conducting power of the walls of a volcanic shaft, there is 

 scarcely any loss of heat from any cause except its enormous absorp- 

 tion in the latent heat of the prodigious volume of dry steam which is 

 constantly being evolved. It is perfectly transparent for several yards 

 above the orifice of the shaft, and is not only perfectly dry steam, but 

 also superheated ; and although this steam may be at the mouth very 

 much below the highest temperature of the hottest point, the temper- 

 ature of the shaft or duct that carries it off will be very nearly at all 

 depths the same, to probably within a very short distance of the point 

 of the greatest incandescence. In the absence, at present, of better 

 information, we may suppose the temperature of volcanic cavities in 

 this region (where Vesuvius is the most ' glaring instance') to be about 

 2000 "Fall. This would give a superior limit of temperature for the 

 interior of our seismic focal cavity, two and one-fifth times as great as 

 the maximum arrived at, by applying the supposed law of hypogeal 

 increment, and would raise the tension of the contained steam (admitting 

 that we know anything about the state of water at such temperatures 

 and pressures) to much more than that due to "fired gunpowder. The 

 capability of producing an earthquake impulse depends greatly, how- 

 ever, upon the suddenness with which the steam is flashed off, and its 

 tension brought to bear upon the walls of the cavity : and this is not 

 most rapid at the highest temperature of the evaporating surface, un- 

 less, indeed, intense pressure, by bringing the fluid more completely into 

 contact with the walls of the heated cavity, may modify the effects of 

 the spheroidal state. On the other hand, the experiments of Boutigny 

 and others indicate that the most sudden production possible of steam 

 would take place from the walls of a focal cavity heated to about 500 

 or 550, which is but a few degrees below that of the mean focal depth 

 as ascertained, namely, 582 Fah." 



The rate at which earthquake-waves travel has always been a sub- 

 ject of great interest and doubt. Mr. Mallet's investigations have, 

 however, so far decided the question, that we may safely assume the 

 following data to be correct. The extreme velocities of the Neapolitan 

 earthquake-waves he found to be 1000 feet per second through substan- 

 ces highly adapted to transmit the waves without much opposition, 

 and 700 feet per second through bad transmitting materials. It is es- 

 sential to remark, that these velocities represent the transit of the wave 

 upon the surface. The velocity of the wave at its maximum, or that 

 of the wave of shock, is considerably less. Mr. Mallet found it to aver- 

 age twelve feet per second, which also agrees with the Holyhead experi- 

 ments. Indeed, calculating from the data published by Humboldt of 

 the celebrated Kiobarnba earthquake, probably the most violent of 



