WHAT IS KNOWN OF THE EARTH. 813 



around it, that the whole solar system is formed of the same kinds 

 of matter, and is subject to the same general physical laws. These 

 conclusions further support the supposition that the earth and 

 other planets have been formed by the aggregation of matter once 

 diif used in space around the sun ; that the first consequence of 

 this aggregation was to develop intense heat in the consolidating 

 masses ; that the heat thus generated in the terrestrial sphere was 

 subsequently lost by radiation ; and that the surface at length 

 cooled and became a solid crust, inclosing a nucleus of much 

 higher temperature. The heat of the interior of the globe in- 

 creases about 1° Fahr. for every fifty or sixty feet of depth below 

 the surface. The surface appears to have now reached a tempera- 

 ture which is virtually fixed, the gain of heat from the sun being 

 just compensated by the loss from radiation into surrounding 

 space. As the exterior gradually cooled, contractions necessarily 

 ensued, producing change of form and dimensions ; and to these, 

 acting in combination with gravity, are, no doubt, largely due the 

 great irregularities of the earth's surface. The strains set up by 

 these forces must have continued to cause movements for a vastly 

 prolonged period, and are doubtless still in action. But the irreg- 

 ularities of the surface constitute only a small part of the effects 

 of internal heat on the earth, and mineralogy is the branch of sci- 

 ence to which reference must be made for a knowledge of the 

 many simple and compound substances that have issued, under 

 the operation of chemical forces, from the vast laboratory con- 

 tained within the cooling crust of the once incandescent globe. 



During the passage of the globe to its present state many won- 

 derful changes must have taken place. The ocean, after its con- 

 densation from a gaseous state into that of liquid, must have long 

 continued in a state of ebullition, or bordering on it, surrounded 

 by an atmosphere densely charged with watery vapor. Apart, 

 however, from the movements in the solid crust of the earth 

 caused by its gradual cooling and contraction, its early higher 

 temperature hardly enters directly into any of the considerations 

 that arise in connection with its present climate ; and it must re- 

 main doubtful how long and to what extent those conditions of 

 climate which interest us most, as having occurred during the 

 period in which the existence of life is indicated, have been af- 

 fected by such early higher temperature. 



In the absence of any direct means of ascertaining the con- 

 dition of the earth's interior, aid has been sought from mathe- 

 matical science, by which it has been established that the thick- 

 ness of the solid outer shell of the earth must be considerable ; 

 and that if the interior is in a fluid state at all, which is very 

 doubful, it must be covered by a great thickness (probably not 

 less than several hundred miles) of solid, comparatively unyield- 



