between this surface and the outer one. the effective thickness of the shell.
The degree of solidity or. fluidity at any point in the interior of the earth must depend partly on the temperature at that point, and may also depend partly on the pressure there. Both causes are here assumed to be effective : if the latter be not so, it will easily be seen that the conclusion arrived at will, a fortiori^ be true.
If through any point in the interior of the earth, (as, for instance, a point in the axis of rotation,) we take a surface of equal tempera- ture, and through the same point, a surface of equal pressure, it is evident that the surface of equal fluidity (or solidity) through that point must be intermediate to these two surfaces. Its exact posi- tion cannot be determined without an experimental knowledge, which we do not possess, of the relative effects of temperature in opposing, and of pressure in promoting the process of solidification. It is sufficient, however, for the purpose now in view, to know that it must necessarily lie between the surfaces of equal temperature and of equal pressure as its extreme limits ; and of these the author pro- ceeds to determine the position.
The forms of the isothermal surfaces within a spheroid have never been completely determined. The determination given by the au- thor is an extremely approximate one when the ellipticity is small, and the time during which the process of cooling has been going on is very great, as it is presumed to be in the case of the earth.
The author then enters into the analytical investigation of this problem ; and deduces the conclusion that we must descend to a depth greater than about one-fifth of the earth's radius before we arrive at a surface of equal fluidity (or solidity) having an ellipticity of the requisite value : that is, the effective thickness of the crust must be at least equal to one-fourth or one-fifth of the earth's ra- dius, in order that the precession may have its observed value : a conclusion, the author observes, which entirely removes the founda- tion of certain vague and somewhat fanciful speculations in geology, proceeding on the hypothesis of the thickness of the earth's crust not being greater than twenty or thirty miles. It has been imagined that in active volcanos, the volcanic vent may communicate directly with the central fluid nucleus, whence the ejected fluid mass has been supposed to be derived. This notion, the author conceives, is rendered totally inadmissible, when it is proved that the thickness of the solid portion of the globe cannot be less than 800 or 1000 miles. It is also remarked, that it follows from the great thickness of the crust, that the present interior temperature of the earth cannot be due to its original heat unless pressure be effective in promoting so- lidification, a fact not yet established by experiment : for, if the present temperature be due to that cause, it is certain that it must be sufficient at the depth of probably less than fifty miles to reduce the matter composing the crust of the globe to a state of fusion un- der the atmospheric pressure ; whereas it has been proved that the earth is solid to a very much greater depth ; which can be account-
