204 SEE— TEMPERATURE, SECULAR COOLING [April 20, 



convection currents, and thus before encrustation of nearly uniform 

 temperature throughout. This hypothesis Hes at the basis of Lord 

 Kelvin's famous paper on the secular cooling of the earth. But in 

 more recent times the conviction has grown that the temperature 

 must have been a maximum at the center, and a minimum at the 

 surface. This law of temperature distribution has been found to 

 be true of all gaseous masses when subjected to strict mathematical 

 inquiry (cf. A. N., 4053, 4104) ; and it is natural to think that a 

 similar law holds for planets even after they have become encrusted. 



Lane first arrived at such views, from his researches on the theory 

 of the sun, in 1869; ^"^ subsequent investigators agree that the 

 density, pressure and temperature would be highest at the center 

 (cf. A. N., 4053, 4104). In the paper on the *' Physical Constitu- 

 tion of the Heavenly Bodies" (A. N., 4053) reasons are assigned 

 for holding the view that all bodies of considerable size must become 

 monatomic by dissociation of the more complex molecules into atoms 

 at their maximum temperature ; and curves are given for the density 

 and temperature corresponding to this condition. It is also shown 

 that after passing through the monatomic condition, encrustation 

 finally results from restricted circulation and surface cooling, which 

 causes great increase of the surface density. If this view be admis- 

 sible, it will follow that the central temperature is not materially 

 lowered by the cooling incident to the formation of the crust. 



We shall therefore be justified in concluding that the monatomic 

 distribution of temperature still holds true approximately in an en- 

 crusted planet. But we have already given reasons why the tem- 

 perature curve, in the case of a body like the earth, which may 

 possibly be of a meteoritic origin, has nearly the form of an ellipse. 

 If we look at the curve for a monatomic distribution of temperature, 

 given in the accompanying plate, right hand upper corner, and imag- 

 ine that the surface density increases as encrustation advances and 

 the heat flows outward, we shall see that the monatomic curve may 

 easily pass into an ellipse such as was postulated for an encrusted 

 planet. Assuming the earth to be encrusted, and the internal tem- 

 perature in any section to follow the law of an ellipse changing w^ith 

 the time, the question naturally arises : What would be the changes 

 produced by the steady flow of heat under the conditions assumed 

 by Fourier? 



