706 
Some Results of Gravitation. 
[Decembe 
until, at a considerable depth, the two increments of 
attraction may become equal, and a region — or a spherical 
shell — of no effective gravity be reached. Beyond this 
region the gravity may possibly become reversed, and the 
pressure of matter be upwards, instead of downwards. 
Going still deeper, and entering the gaseous region, this 
reversing influence would gradually diminish with descent, 
as the neighbouring dense matter was receded from, and the 
distant dense matter approached. Finally, at the centre, 
another region of no gravity would be found, the opposite 
attractions being again balanced. 
Thus there would be a surface shell of downward gravity, 
at first increasing, and then decreasing to an interior limiting 
shell of no effective gravity ; beyond this a deeper region of 
upward gravity, which again, after reaching its limit of 
effectiveness, slowly decreases, and merges in a second region 
of no effective gravity at the centre. Of course this hypo- 
thesis rests on the assumption that the earth consists of a 
thick shell of solid matter surrounding a, perhaps much 
thicker, gaseous interior. In such a case the deductions 
above presented seem beyond question. And that such is 
the interior constitution of the earth the necessary behaviour 
of heat in a condensing nebula certainly renders probable. 
If condensation was confined to the superficial region 
until considerable density was produced, there would cer- 
tainly be a reduction of the centripetal gravity interiorly. 
And if the superficial condensation reached a certain degree 
ere making its effeCt felt in the deep interior, then the 
gravity of the rare material adjacent to the inner dense sur- 
face must necessarily be so reduced by upward attraction 
as to be finally negatived. But compression would not 
cease with the production of this region of no effective 
gravity. Although no longer aided by gravity, the pressure 
of the dense material above must still have made itself felt, 
and produced condensation below this point. This would 
tend to produce the reversal of gravity above considered, and 
gravitative compression would now be upward instead of 
downward. Its tendency, therefore, would be not to augment, 
but to decrease the centripetal downward compression of the 
interior gas, and there must result a compression proceeding 
from centre to surface, not from surface to centre. The gas 
would become densest at the region adjoining the solid crust, 
and rarest at the centre. But such a process could not in- 
definitely continue. The growing rarity of the remaining 
gas as a portion of it became condensed, and its decreasing 
temperature in consequence, must necessarily adt as a limiting 
check to condensation, and yield a final equilibrium. 
