40 PRECESSION OF THE EQUINOXES AND NUTATION. 



assuming a uniform primitive melting temperature of 7000° Fall., and a lapse of 

 100 millions of years since the cooling process connnenced. 



"The rate of increase of temperature from the surface downwards Avould be sen- 

 sibly ^\ of a degree per foot for the first 100,000 feet or so. Below that depth the 

 rate of increase per foot would begin to diminish sensibly. At 400,000 feet it 

 would liave diminished to about jl^ of a degree per foot. At 800,000 feet it would 

 have diminished to less tlian ^V of its initial value, that is to say, to less than ^^V^ 

 of a degree per foot; and so on, rapidly diminishing. Such is, on the Avhole, the 

 most probable representation of the earth's present temperature, at depths of from 

 100 feet, where the annual variations cease to be sensible, to 100 miles, below 

 which the wliole mass, or all except a nucleus cool from the beginning, is (whether 

 liquid or solid) probably at, or very nearly at, the proper melting temperature for 

 the pressure at each deptli." 



The high rigidity demanded is difficult to conceive of in connection with a 

 temperature in the solidified mass " at or near" that of melting, extending down- 

 wards indefinitely towards the centre of the earth. Hence Poisson's reason- 

 ing, wliicli results in showing that the earth to have become thoroughly cooled and 

 to have been subsequently reheated, svperfickdJij, harmonizes better with the 

 demand for rigidity than that of Leibnitz, which supposes it to be now cooling 

 from a (throughout) incandescent liquid state. In the latter case the law of actual 

 temperature as deduced from Tourier's formula? (as expressed above by Sir W. 

 Thomson) would extend to the centre; in the former case only to the unmelted 

 portion or to the "nucleus cool from the beginning." 



lleferring to the increase of temperature, with depth observed in mines, &c., 

 Poisson remarks: "^Fourier et ensuite Laplace out attribue ce phenomene a la 

 chaleur d'origine que la terre conserverait a I'epoque actuelle et qui croitrait en 

 allant do la surface an centre, de tel sort qu'elle fut excessivement elevee vers le 

 centre * * * en vertu de cette chaleur initiale la temperature serait aujourd'hui 

 de plus de 2000 degres a une distance de la surface egale sculemcnt an centieme 

 du rayon ; au centre elle surpasserait 200,000 degres. * * * Mais quoique cette 

 explication ait ete generalement adoptee, j'ai expose, dans mon ouvrage, les difficultes 

 qu'elle presente, et qui m'ont paru la rendre inadmissible, * * * je crois a\oir 

 demontrc que la chaleur developpee par la solidification dc la terre a du se dissiper 

 pendant la duree de ce phenomene, et que de];)uls loi/gfemjis il n' en svhsisic 2>hts 

 aucnne trace." (Theorie dc la Chaleur.) 



Poisson, as is well known, attributes the increase of temperature, with depth, 

 observed in the earth's crust, to the passage, at a remote period, of the solar system 

 through hotter stellar regions, the temperature of which, he argues, should differ 

 from place to place. Even a hypothetical case of the illustrious author, conform- 

 mg to his theory, of an increased supei-ficial temperature, 5000 centuries ago, of 

 200° C. diminishing by cooling (by transition to cooler stellar regions) to 5°, 500 

 centuries ago, and, subsequently, to the actual mean temperature, would scarcely 

 meet the present demands for time, of palfcontolo<;v ; while the determinations of 

 the conductivity of the earth's crust made near Edinburgh show, according to Sir 

 ^^. 'Ihunison, a necessity for increments of temperature of 25°, 50°, arid 100° 



