260 



EECEEATIYE SCIENCE. 



some great and general cause, sucli only as 

 the hypothesis of the central heat of the 

 earth affords us. That hypothesis supposes 

 the earth to contain within it a mass of heated 

 material ; nay, it supposes the earth to be a 

 heated and incandescent body, habitable only 

 because surrounded with a cool crust — the 

 crust being to it a mere shell, within which 

 the vast central fires are securely inclosed. 

 And yet not securely, perhaps, unless such 

 vents existed as those to which we apply the 

 term volcano ; and, according to the hypothe- 

 sis of the central heat of the earth, every 

 volcano is a safety-valve, ready to relieve the 

 pressure from within when that pressure rises 

 to a certain degree of intensity, or perma- 

 nently serving for the escape of conflagra- 

 tions, which, if not so provided with escape, 

 might rend the habitable crust to pieces. 



Passing to earthquakes, the fact which 

 first strikes us is their intimate connection 

 with volcanoes. Shocks of earthquake usually 

 accompany volcanic fires, and emissions of 

 sulphureous fumes, watery vapour, and even 

 mud, sometimes accompany the occurrence of 

 these convulsions. This was the case in 

 the great earthquake which happened in 

 SicUy, in October, 1835, and which travelled 

 100 miles in the space of half an hour. In 

 the destruction of Cartago, in Central Ame- 

 rica, in January, 1842, the centre of the 

 shock was in the midst of a region fruitful in 

 volcanic phenomena, and only three leagues 

 from a constantly active crater. The whole 

 of the West Indian islands are composed of 

 volcanic soil, and constitute a portion of a 

 great volcanic series, and these islands are 

 constantly agitated by the throes of earth- 

 quake. The earthquake of Lisbon was one 

 of a series of shocks which disturbed the 

 wliole extent of the volcanic region of the 

 south of Europe, and reached even to Africa, 

 every portion of the region affected having 

 its own volcanic centre, active or extinct. 

 Surely chemical causes will not be sought to 

 produce concussions which extend over hun- 



dreds of square miles of surface, the extreme 

 points of which are agitated so nearly at the 

 same moment! If electrical agencies were 

 referred to, no doubt many of the conditions 

 required would be fulfilled ; but if the inte- 

 rior of the globe is a molten mass of fiery 

 fluid, which occasionally breaks forth and 

 manifests itself at the surface, electrical and 

 magnetic disturbances are to be expected as 

 concomitant phenomena ; and these, when 

 once roused into action, as indeed they must 

 be by such extensive and sudden convulsions, 

 would have power to direct, control, and 

 even to impress some of their own charac- 

 teristics on the general aspect of the convul- 

 sion. That they have done so there can be 

 no doubt. For four years preceding the ter- 

 rible outbreak at Lisbon, the electrical dis- 

 turbances were marked and frequent; tre- 

 mors agitated the earth, and many geological 

 changes took place in the arrangements of 

 the strata through the excessive drought, 

 which reduced the conducting power of the 

 soil. Indeed, on the theory of central heat, 

 drought alone, if sufficiently intense, would 

 produce earthquakes when the rains occurred 

 again ; for, as the strata differ as to their 

 capacity for water, some would have their 

 conducting power renewed before others, and 

 thus become a sort of insulated conductors. 

 If the electric fluid accumulated in the air 

 sufiiciently to discharge itself abundantly to 

 the earth, a succession of shocks would occur. 

 These shocks would reopen the cracks in the 

 earth's crust in a region where many such 

 cracks must abound, water would trickle 

 through from the prevalence of heavy rain ; 

 this reaching the incandescent mass below, 

 would generate steam ; and then, after the 

 preliminary rumblings caused by the electric 

 fluid, would come the great and terrible ex- 

 plosion, crushing in a few moments the monu- 

 ments of a thousand years of human toil and 

 enterprise and suffering, and changing cities, 

 hamlets, and fertile fields into frightful de- 

 serts, more rapidly than the light of the suij 



