REPORT OF THE CHIEF ASTRONOMER 697 



SESSIONAL PAPER No. 25a 



tures, from a remote time when the planet's diameter was considerably less than 

 at present. The continued accretion of planetesimals has since enlarged the 

 Earth. The compression of the interior of a planet constantly solid is supposed 

 to have generated heat sufficient to explain vulcanism and the observed thermal 

 gradient near the Earth's surface. 



'As the conduction of heat through rock is exceedingly slow, the cen- 

 tral heat may be assumed to have continued to rise so long as the infall of 

 matter caused appreciable compression. In the same way, heat was gener- 

 ated progressively in the less central parts, and these parts also received 

 the heat that passed out from beneath. It is assumed under this hypothesis 

 that the degree of interior compression stands in close relation to in- 

 terior density, for while there would probably be some segregation of 

 heavier matter toward the center and of lighter toward the surface by 

 means of volcanic action and internal rearrangement under stress differ- 

 ences, the interior density is regarded as due mainly to compression. The 

 distribution of internal pressure and density generally accepted is that of 

 Laplace, who assumed that the increase of the density varies as the square 

 root of the increase of the pressure. This law gives a distribution of 

 density that accords fairly well with the phenomena of precession of the 

 equinoxes, which require that the higher densities of the interior shall be 

 distributed in certain proportions between the center and the equatorial 

 protuberance whose attraction by the sun and moon causes precession. The 

 increases in pressure, density and temperature have been computed as 

 follows by Mr. A. C. Lunn, the average specific gravity of the earth being 

 taken at 5-6, the surface specific gravity at 2-8, and the specific heat at 

 .9 



' The accretion hypothesis assumes that, during the growth of the earth, 

 large amounts of heat were carried by volcanic action from deeper horizons 

 to higher ones and to the surface, and that this still continues at a dimin- 

 ished rate. It assumes that whenever the interior heat raised any constituent 

 of the interior matter above its fusing-point under the local pressure, it 

 passed into the liquid state, and was forced outwards by the stress differences 

 to which it was subjected, unless its specific gravity was sufficiently high 

 to counterbalance them. It is conceived that the more fusible portions 

 were liquefied first, and that in so doing they absorbed the necessary heat 

 of liquefaction and began to work their way outward, carrying their heat 

 into higher horizons and temporarily checking the development of more 

 intense stresses in the lower horizons. They thus served to keep the tem- 

 perature there below the fusion-point of the remaining more refractory 

 substances. Meanwhile the extruded portions were raising the temperatures 

 of the higher horizons into which they were intruded or through which 

 they were forced to pass.'* 



*T. C. Chamberlin and R. D. Salisbury, "Geology," New York, 1906, Vol. I, pp 

 564-567. 



