EESISTANOE TO VOLCANIC ACTION. 611 



tliis opinion is sound. Tlie earthquake of the first of November,. 

 1755, which is asserted, though upon doubtful evidence, to have 

 been felt over a twelfth part of the earth's surface, was among 

 the most violent of which we have any clear and distinct account, 

 and it seems to have exerted its most destructive force at Lisbon. 

 It has often been noticed as a remarkable fact, that the mint, a 

 building of great solidity, was almost wholly unaffected by the 

 shock which shattered every house and church in the city, and 

 its escape from the common ruin can hardly be accounted for 

 except upon the supposition that its weight, compactness and 

 strength of material enabled it to resist an agitation of the earth 

 which overthrew all weaker structures. On the other hand, a 

 stone pier in the harbor of Lisbon, on which thousands of peo- 

 ple had taken refuge, sank with its foundations to a great depth 

 dm-ing the same earthquake ; and it is plain that where subter- 

 ranean cavities exist at moderate depths, the erection of heavy 

 masses upon them would tend to promote the breaking down of 

 the strata which roof them over. 



No physicist, I believe, has supposed that man can avert the 

 eruption of a volcano or diminish the quantity of melted rock 

 which it pours out of the bowels of the earth ; but it is not al- 

 ways impossible to divert the course of even a large current of 

 lava. " The smaller streams of lava near Catania," says Ferrara, 

 in describing the great eruption of 1669, "were turned from 

 their course by building dry walls of stone as a barrier against 



them It was proposed to divert the main current from 



Catania, and fifty men, protected by hides, were sent with hooks 

 and iron bars to break the flank of the stream near Belpasso.* 



* Soon after tlie current issues from the volcano, it is covered above and at 

 its sides, and finally in front, with scoriae, formed by the cooling of the ex- 

 posed surface, which bury and conceal the fluid mass. The stream roUs on 

 under the coating and between the walls of scoriae, and it was the lateral crust 

 which was broken through by the workmen mentioned in the text. 



The distance to which lava flows, before its surface begins to solidify, de- 

 pends on its volume, its composition, its temperature and that of the air, the 

 force Avith which it is ejected, and the inclination of the declivity over which 

 it runs. In most cases it is difflcult to approach the current at points where 

 it is still entirely fluid, and hence opportunities of observing it in that condi- 

 tion are not very frequent. In the eruption of February, 1850, on the east 

 side of Vesuvius, I went quite up to one of the outlets. The lava shot out of 



