NATURAL PHILOSOPHY. 173 



and by analogy it may be concluded that such would, at least in some con- 

 siderable degree, be the case with the mineral matter of the earth's crust. 

 The chalk is that formation in which the most numerous and some of the 

 best observations on terrestrial temperatures have been made; and it would 

 seem impossible to conclude, from actual experiment and the considerations 

 above stated, that its conductive power can exceed one-third of that of the 

 inferior racks, and may not improbably be a considerably smaller fraction 

 of it. Xow the increase of depth in the chalk corresponding to an increase 

 of 1 Fah. is well ascertained to be very nearly GO feet, and therefore the 

 rate of increase in the inferior rocks must probably be at least three times 

 as great as in the chalk, and may be very considerably greater still. Hence, 

 supposing the thickness of the solid crust would be about (iO miles if the 

 conductive power of its lower portion were equal to that of chalk, its actual 

 thickness must probably be at least about 200 miles, and may be consider- 

 ably greater, even if we admit no other source of terrestrial heat than the 

 central heat here contemplated. There is also another way of investigating 

 the thickness of the earth's crust, assuming the whole terrestrial mass to 

 consist of a fluid nucleus inclosed in a solid envelope. If the earth were 

 accurately spherical, instead of being spheroidal, its axis of rotation would 

 always remain exactly parallel to itself, on the same principle as that on 

 which the gyrascope preserves, in whatever position it may be held, the 

 parallelism of the axis about which it rotates. But the attraction of the 

 sun and moon on the protuberant equatorial portions of the earth's mass, 

 causes a progressive change in the position of the earth's axis, by virtue of 

 which the north pole, or that point in the heavens to which the northern 

 extremity of the earth's axis is directed, instead of being stationary, de- 

 scribes a circle on the surface of the heavenly sphere about a fixed point in 

 it, called the pole of the ecliptic, with a radius of nearly 23j, equal to the 

 inclination of the equator to the ecliptic, or the obliquity. The whole of this 

 revolution is completed in about 2o,000 years; but, as follows from what 

 has just been stated, without any change, beyond small periodical ones, in 

 the obliquity. A corresponding change of position must manifestly take 

 place also in the position of equinoxes, which have thus a motion along the 

 ecliptic in a direction opposite to that in which the signs of the zodiac are 

 reckoned, completing a revolution in the period above mentioned of 2-1,000 

 years. It is called the precession of the equinoxes. This processional motion 

 has been completely accounted for under the hypothesis of the earth's entire 

 solidity, and that of a certain law according to which the earth's density 

 increases in approaching its centre; but some years ago Mr. Hopkins inves- 

 tigated the problem with the view of ascertaining how far the observed 

 amount of precession might be consistent with the existence of a fluid nu- 

 cleus. The result was, that such could only be the case provided the thick- 

 ness of the solid shell were much greater than that which, as above stated, 

 has been supposed by many geologists. The numerical result was that the 

 least admissible thickness of the crust must be about one-fifth of the earth's 

 radius ; but, without assigning any great importance to an exact numerical 

 result, Mr. Hopkins had a full confidence in the investigation, as showing 

 that the thickness of the crust could not be so small as 200 or 300 miles, and 

 consequently that no geological theory can be admitted which rests on the 

 hypothesis of the crust being nearly as thin as it has been frequently as- 

 sumed to b"e. The influence of the interior fluidity on the processional 

 motion above described, is due to the difference between the motions which 



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