35 



mutual attraction of the particles of an irregular nebulous 

 mass there result condensation, rotation, heat, and light. 



It follows as a corollary from the Nebular Hypothesis, 

 that the Earth must at first have been incandescent ; and 

 whether the Nebular Hypothesis be true or not, this origi 

 nal incandescence of the Earth is now inductively established 

 —or, if not established, at least rendered so highly pro- 

 bable that it is a generally admitted geological doctrine. 

 Let us look first at the astronomical attributes of this once 

 molten globe. From its rotation there result the oblate- 

 ness of its form, the alternations of day and night, and (un- 

 der the influence of the moon) the tides, aqueous and at- 

 mospheric. From the inclination of its axis, there result 

 the precession of the equinoxes and the many differences of 

 the seasons, both simultaneous and successive, that pervade 

 its surface. Thus the multiplication of efiects is obvious. 

 Several of the dififerentiations due to the gradual cooling 

 of the Earth have been already noticed — as the formation 

 of a crust, the solidification of sublimed elements, the pre- 

 cipitation of water, &c., — and we here again refer to them 

 merely to point out that they are simultaneous efi^ects of 

 the one cause, diminishing heat. 



Let us now, however, observe the multiplied changes 

 afterwards arising from the continuance of this one cause. 

 The cooling of the Earth involves its contraction. Hence the 

 solid crust first formed is presently too large for the shrink- 

 ing nucleus ; and as it cannot support itself, inevitably follows 

 the nucleus. But a spheroidal envelope cannot sink down 

 into contact with a smaller internal spheroid, without disrup- 

 tion ; it must run into wrinkles as the rind of an apple does 

 when the bulk of its interior decreases from evaporation. 

 As the cooling progresses and the envelope thickens, the 

 ridges consequent on these contractions must become 

 greater, rising ultimately into hills and mountains ; and the 

 later systems of mountains thus produced must not only be 



