236 Wisconsin Acadeinij of Sciences, Arts and Letters. 



VIII. 



Ine Figure of Equilibrium, the Denfiity and Temperature of the Earth and 



Other Planets. 



First. — The ohlate Ellipsoid is the figure of equilihrium of a rotating fluid 

 mass of heterogeneous density, and all solids manifest the fluid 

 property in some degree under a requisite continued pressure. 



Second. — The average density of the earth's crust in depth about 130 miles 

 is 2.96 tiines water, and at the center about 3.05 times that of the 

 crust, or 9 times water. At other points the density is given in 

 table 4, this chapter. 



Third. — The temperature of the earth from surface to center at first in- 

 creases about 1° C. per 90 feet, but this rate of increase so 

 diminishes that at the depth of 130 miles the temperature 

 becomes about 3,600'" C. Thence to the center the rate of increase 

 is so small that the temperature continues nearly uniform. 



Fourth. — The earth has yet to cotract, radially, about 150 miles to become 

 throughout zero temperature. The interior flames will be, con- 

 tinually, fed by the fuel of pressure of outside load to a limit 

 beyond which the earth's mass cannot compress. The p resent 

 interior, at a few miles beloiv the surface begins to get p)lastic, 

 and from about 130 miles below the surface to the center it 

 becomes so plastic that the temperature is kept nearly uniform 

 by convection of heat. 



Note. — The density of a fluid earth composed, throughout, proportion- 

 ally of the same elements would increase from surface to c 'uter, not with- 

 standing the comm">tion produced by the heat generated by pressure. In 

 case of such an e^rth at 15° C, if there were no heat from pressure, and 

 therefore a state of rest, its increase in density would be per law of com- 

 pression. If heat sufficient to raise the temperature of the whole mass 

 50° C. were put into such earth so as to leave surface at 15° C, and have 

 the increase in tempergture from surface to center per law of compression, 

 then in case no heat escaped, per law of convection there would be no state 

 of rest tili the whole mass became of uniform temperature, 65' C. It is 

 evident, then, that an earth having a crust with moulten interior would 

 be at a state of I'est proportionally to the thickness and non-conducting 

 heat property of its crust. Not with-standin? the moulton interior of the 

 earth (if she has such) is now at temperature nearly unifoi'm, yet I sup- 

 pose before the crust solidified or become plastic that heat radiated into 

 cold space so rapidly thit in spite of the law of convection, the tempera- 

 ture at the center of the earth was thousands of degrees higher thaa at 

 the surface. The heighth to which material is now being thrown from 

 the sun, proves that the temperature at his center is higher beyond com- 



