PAGE 184 Inquiry into the interior construction of the earth--_continued_. 185 Density of 8·8 times that of water still too high for the possible compression of the component matter of the earth as known to us. 186 Reasons for this conclusion drawn from crushing strains of materials. 187 A limit to density shown thereby. 188 The greatest density need not exceed 6·24 of water. 189 Gases shut up in the hollow centre. Their weight must so far diminish the conceded maximum of 6·24. 190 Density of inner half of earth at 3000 miles diameter. Greatest density may be less than 5·833 of water. 191 Supposed pressure of inclosed gases very moderate. 193 Meaning of heat limit to density. Temperature of interior half of shell and inclosed gases must be equal. 194 State of the hollow interior. 195 Results of the whole inquiry.
INQUIRY INTO THE INTERIOR CONSTRUCTION OF THE EARTH--_continued_.
It may be well to revert here to the experiment we made of putting a cubic foot of rock, of specific gravity 13·734 in the scale of a balance at the centre of the earth, where we saw that it could not depress the scale one hair-breadth, and make the same experiment by placing a cubic foot of rock of 8·8 specific gravity in the same scale, at what we have called the region of greatest density of the earth, that is, at 817 miles from its surface. Here, also, we shall find that the scale is not depressed for the very same reason as in the former case, that is because it had nowhere to be depressed to; and it might be argued that for the same reasons advanced formerly there can be no matter at that place, but the cases are entirely different. In the first case, there is nearly the whole mass of the earth drawing the matter away from the centre were it at liberty to move; whereas, in the second case, the meeting of the two halves of the shell, at the region where there is the greatest mass of matter, is also the meeting place of the action of attraction in its greatest force; the place to which matter is attracted from all sides, remains stationary, and it is held there both by attraction and weight of superincumbent matter or gravitation. The attraction of the whole earth acts as if it were concentrated at its centre, but that is for external bodies. That kind of attraction on the inner half of the shell would be far inferior to that outwards of the outer half, owing to its greater distance and conflicting nature, and would perturb, as we have said, but not do away with it. The same could not occur at the centre, because it is not the centre of the mass, that is, it is not the place where the greatest quantity of matter existed originally, or is now to be found, and consequently never was, nor can now ever be, the actual centre of interior attraction.