0. Fisher — Rival Theories of Cosmogony. 415 



earth, there are two things about which we may feel tolerably 

 certain, and these are, first, that the lawof internal density is 

 fairly represented by what is known as Laplace's law, which 

 depends upon the assumption that the increase of the square 

 of the density varies as the increase of the pressure. Hence 

 we can decide approximately what the density will be at any 

 given depth. The other fact, upon which we may feel assured, 

 is the high internal temperature of the earth. It is proposed 

 to enquire how these facts may severally be accounted for on 

 the meteoric theory. 



Let us consider first the question of densities. If the earth 

 was built up of meteorites, these may be expected to have 

 come in promiscuously — not all the heavy ones first and the 

 lighter last. We may provisionally assume that their average 

 density may have been nearly that of the present surface den- 

 sity, usually taken as 2*75. If this is too low, the arguments 

 based upon it will not be affected in any great degree. 



The high central densities may then be due to one of two 

 causes, to compression, or to a higher intrinsic density owing 

 to a rearrangement of the material. And first of compression. 



In the Appendix to my " Physics of the Earth's Crust," I 

 have said that, " In order to form an idea of the nature of the 

 matter of the interior which would cause density to vary 

 according to Laplace's law, let us suppose a globe unaffected 

 by gravity composed of layers of varying compressibility, but 

 of density everywhere the same as that of surface rock, and 

 then suppose gravity to act upon it, and to reduce it to the 

 size and density of the existing earth. This will give an idea 

 of the law of compressibility of matter of surface density at 

 various depths, which would be suitable to sustain the pres- 

 sure, so as to bring the law of density into accordance with 

 Laplace's law."* 



Since the pressure and density at a given depth are fixed 

 quantities, it follows that the compressibility at that depth 

 must have a special value if the pressure there is to reduce 

 matter of surface density to the required local density. For 

 if the compressibility is too great the matter will become too 

 dense, and if it is too small it will not become dense enough. 

 The result which I obtained shows that the compressibility 

 must decrease rapidly as the density increases, for, if the com- 

 pressibility were to remain the same all the way down, the 

 material in the lower depths would be very much more dense 

 than, according to Laplace's formula, it actually is. My formula 

 shows that in order to obtain this special compressibility meas- 

 ured in atmospheres per square foot at the depth where the 

 density is p, we have merely to divide 3*6069 X 10" 5 by the 



* Appendix to "Physics of the Earth's Crust," p. 30, 1891. 



