i88i.] Some Results of Gravitation, 701 
gravity must decrease in vigour downwards, and become 
null at the centre. 
The heat contents of such a nebular mass may be consi- 
dered as absolute heat, the temperature being to a certain 
degree uniform. But the effeCt of gravitative condensation, 
either direCtly through attraction or indirectly through com- 
pression, necessarily rendered some of the latent heat sensible, 
and thus increased the temperature of the condensed regions 
of the nebula. The resistance to compression would be the 
same whether this heat existed as sensible or as latent heat ; 
but as soon as it became sensible the disturbance of temper- 
ature equilibrium introduced a new element into the case. 
The heat commenced to flow from the denser to the rarer 
portions of the nebula, thus tending to reproduce tempera- 
ture equilibrium, but to overthrow the original heat equili- 
brium. As compression, with its disturbance of temperature, 
must have first made itself felt near the surface, the heat 
yielded would aft to hinder its extension downward. This 
heat must be first got rid of ere compression could effectively 
descend. But the getting rid of such heat would be a very 
slow process. The principal agency in this process would 
be heat convection, or the movement of the heated matter. 
There may have been radiation also, if the nebulous matter 
was transparent. 
Such a loss of heat must have been most rapid at the 
surface, where gravity was greatest and radiation most free 
to aft. The chilled surface matter, becoming heavy, would 
sink ; the heated internal matter, becoming light, would rise. 
Thus there would be a double conveCtion, that of heated 
matter upwards and of chilled matter downwards, and heat 
would be got rid of by both processes, being radiated from 
the surface, and also employed in re-heating the chilled 
matter. By a continuation of these processes the com- 
pressing effects of gravity must have made themselves more 
and more deeply felt, the resisting heat being gradually dis- 
posed of. Had the aCting matter continued in the gaseous 
condition the above process might have continued until com- 
pression was fully produced internally, and there was a 
regular increase of density downward to the centre. 
But long ere this result could have been reached surface 
solidification very probably took place, with the result of 
cutting off the distribution of heat by conveCtion and radia- 
tion from the interior, and leaving only the interminably 
slow process of conduction. Therefore if the full effect were 
not produced before this solidification took place, it would 
be interminably delayed. And another delaying influence 
