EECEEATIYE SCIENCE. 



329 



But if originally fluid, it must, under the in- 

 fluence of rotary motion, assume a figure of 

 equilibrium — a figure expressive of tlie mean 

 between its tendency to cobere, and the ope- 

 ration of centrifugal force to destroy its cobe- 



Exaggerated Outline of the Earth, to show it as an 

 oblate spheroid. 



rence. Sucb a body, having a capability to yield 

 in its outline, would, when made to rotate, be- 

 come depressed to a certain extent in its axis 

 of rotation, and to the same extent expanded 

 at the equator. When the equilibrium had 

 been attained, the mass would remain in the 

 form of an ellipsoid. Now facts just agree 

 with theoretical deductions : the earth has a 

 larger circumference at the equator than at 

 the poles by at least twenty-six miles ; there- 

 fore we may fairly assume that the earth was 

 originally fluid, its fluidity being the result of 

 an intense heat.* 



Let us admit that originally the earth was 

 in an incandescent state ; that in process of 

 countless ages the outer surface cooled by 

 radiation, and so gave rise to the formation of 

 a solid crust ; that within this crust a heated 

 mass of fluid material was inclosed, and that 

 this is not yet sufficiently cooled to assume a 

 solid form. The consequence of the theory 

 wiU be, that we shall observe various evi- 

 dences of the occasional escape of heat from 



* Sir John HersoheL 



within ; that wherever fissures, cracks, or per- 

 forations occur in the shell inclosing the hot 

 material, the escape of fire, the rending of the 

 crust, and other similar phenomena, will be 

 manifested. We have only to take the con- 

 verse view to be assured that boiling springs, 

 earthquakes, and volcanoes are bat several 

 evidences of the existence and occasional 

 escape of internal heat. 



We need not rest, however, with the 

 mere determination of the fact ; we may, with 

 some accuracy, measure its force, its amount, 

 and its comparative duration during time, and 

 even make some sober conjectures aa to its 

 history in the future. 



What is the rate of increase of heat down- 

 wards ? Kupffer estimated the ratio of in- 

 crease at Grenelle to be 1^ for every 37 feet ; 

 the Durham and Northumberland mines give 

 1° for ever 44 feet ; the lead mines of Saxony 

 r for every 65 feet ; and the mine of Dol- 

 bouth, which is 1380 feet in depth, 1° for 75 

 feet. The best estimates, however, are those 

 deduced from a comparison of a large number 

 of results, and these give the rate of increase 

 as being equal to 1'' of Fahrenheit for every 

 45 feet of descent. Looking to the result of 

 such a rate of increase, it is easy to see that 

 at 7290 feet from the surface, the heat will 

 reach 212% the boiling point of water. At 

 25,500 feet it will melt lead ; at 7 miles it 

 will maintain a glowing red heat, at 21 miles 

 melt gold, at 74 miles cast iron, at 97 miles 

 soft iron, and at 100 miles from the surface 

 aU will be fluid as water, a mass of seething 

 and boiling rock in a perpetually molten 

 state, doomed possibly never to be cooled or 

 crystallized. The heat here will exceed any 

 with which man is acquainted ; it will exceed 

 the heat of the electric spark, or the effect of 

 a continued voltaic current. The heat which 

 melts platina as if it were wax is as ice to it ; 

 could we visually observe its efiects, our in- 

 tellect would afibrd no means of measuring 

 its intensity. Here is the region of perpetual 

 fire, the source of earthquake and volcanio 



