puKsiDKxr s adduf:ss. hi. 



Still, many observations prove a close relation between volcanoes 

 and earthquakes. I need not cite many examples, but will 

 merely recall the earthquakes wliicli wrought such havoc in 

 Central America just before the stupendous explosion of 

 Coseguina, ni Nicaragua, in 1835, and the carthrpuikes of 

 gradually increasing violence which preceded tlie famous 

 eruption of Vesuvius, in a.d. 70. It appears pretty certain that 

 the determining cause of a volcanic eruption is the access of 

 water to some mass of melted rock. The origin of the melted 

 matter, whether locally developed or a portion of the still fluid 

 interior mass of the earth, is immaterial to our present purpose. 

 But, given this heated mass, the access of the water would 

 produce violent explosions, which yet might not be enough to 

 do more than shake the superincumbent strata. By shaking 

 these, however, the cracks and channels by which the water 

 sinks into the earth would be enlarged, and the explosive 

 conversion into steam would go on till the force was sufficient to 

 burst the crust at some point of weakness, and so result in the 

 forcing up of the melted mass, either as so-called ashes or in a 

 stream as lava. The fact that in so many cases the judicious 

 examination of the movements produced by the shock, of the 

 siu'face, and of movable objects on it, especially as observed by 

 means of the instruments of precision now constructed for the 

 purpose, indicates that the focus or centre of disturbance is 

 situated at some depth below the surface, would seem to lend 

 some support to this supposition. Thus Mallet determined the 

 depth of the original shock of the earthquake in Calabria of 

 1857 as about 5 miles, and results varying in different cases 

 from 4 to 14^ miles have been obtained in the case of some of 

 the recent European shocks. One of the photographs referred 

 to above showed the direction of the blow in a very remarkable 

 manner, the corner of a building having been thrown off and 

 hurled across a street, wrecking the opposite houses. In 

 " Nature " for September 6th, 1883, p. 438, will be found a map 

 of the island of Ischia, showing lines along which the effects 

 were the same. The examination of these leads to the proba- 



