82 HISTORY or 



But before we proceed with the causes, let us take a short 

 view of the appearances which have attended the most remark, 

 able earthquakes. By these we shall see how far the theorist 

 corresponds with the historian. The greatest we lind in anti- 

 one degree for 45 feet." In the deep borings made by M. Arago, it was found 

 that the greater the depth from whiih the water ascended the wanner it was . 

 Mr Bald of Alh)a published some facta in the Edinburgh Philosophical Jour- 

 nal some years ago, and Mr Daubuisson gives others relating to the mines of 

 Saxony, which establish the same conclusion. A writer in the Annals oj 

 Philosophy thinks that the increase in England is about one degree of P'sli- 

 renheit for every ten or twelve fathoms of descent. Hence we have reason 

 to conclude, that it is not any peculiar local circumstances which generate the 

 heat in warm springs ; but that they merely derive their waters from reser. 

 voirs situated at a great depth. Pursuing this idea, the Bath wateis which 

 have a temperature of 116, may be supposed to come from a depth of three- 

 fourths of a mile, and at the depth of two miles downwards we should find 

 the temperature of boiling water ; and that of melted lava at a depth wliich 

 varies much in ditferent countries, but may be estimated on an average at GO 

 miles. Cordier, therefore, considers the whole globe as a mass effused matter, 

 intensely hot, covered with a solid crust or shell, whose thickness is about 

 one 63d part of its semidiameter, and upon which crust man and all his works 

 are suspended over the molten abyss. Hence M. Cordier considers the quan- 

 tity of matter thrown out by volcanoes, as affording a measure of the contrac- 

 tion which the shell of the globe is tmder going. He has examined and esti- 

 mated the cubic contents of the matter ejected in one eruption by several 

 volcanoes, and found that it did not amount to a cubic kilometre, that is, a 

 cube, the side of which is rather more than half a mile. It follows, that a con- 

 traction of volume, which would shorten the radius of the globe the 50,000th 

 part of an inch, would be sufficient to force out the matter of one eruption. 

 There are about 200 unextinguished volcanoes known, but many of them 

 are silent for very long periods ; but assuming that there are on an average 

 five eruptions in a year among them all, tliis would indicate that tlie 

 radius of the globe shortened only one milliraetre(l-2dth part of an inch) iii a 

 century. 



" This theory," says a journalist of great intelligence, "accounts for the 

 frequency of volcanoes in the early stages of the globe's existence, when the 

 crnst was thin, the contraction rapid, and the fracture of its parts easy. As 

 it increases iu tliickness, changes in its figure or volume become more ditfi. 

 cult, and must be chiefly confined to the inner coats, among which it is pro. 

 hable that void spaces may be left ; into these the fluid matter may be injected, 

 which in earlier times would have reached tlie surface and formed eruptions. 

 Assuming that the thickness of the crust is 60 miles, it would require a 

 pressure equal to that of 28,000 atmospheres to make the fused iKia 

 reach the surface. Hence we see why such a vast number of volcanoes 

 are found every where on the earth's surface, which were once active, but 

 are now extinguished. In early times, when the earth was perfectly fluid 

 at the surface, the attraction of the s:un and moon would produce tides in the 

 molten mass exactly as it does now in the ocean. These tides, which must 

 have been four or five yards in height, would exert a disturbing f'>rce ou the 



