1859.] and the Thickness of its Crust. . 141 



power. These considerations lead to the conclusion that the conduc- 

 tivity of the inferior portions of the earth's solid crust must be much 

 greater, and may be very much greater, than that of the less consoli- 

 dated and mere superficial sedimentary beds. Moreover, the tempera- 

 ture of fusion of certain substances, as Mr. Hopkins had shown by 

 experiment, is much increased by great pressure ; and by analogy it 

 may be concluded, that such would, at least in some considerable 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 obser- 

 vations 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 rocks, and may not improbably be a considerably smaller 

 fraction of it. Now the increase of depth in the chalk corresponding 

 to an increase of 1° (F.) is well ascertained to be very nearly 60 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 consi- 

 derably greater still. Hence, supposing that the thickness of the solid 

 crust would be about 60 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 considerably 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 gyroscope 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, describes 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 23^°, equal to the inclination of the equator to the 

 ecliptic, or the obliquity. The whole of this revolution is completed 

 in about 25,000 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 the 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 

 25,000 years. It is called the precession of the equinoxes. 



This precessional 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 J but some years ago Mr. Hopkins investigated the problem 



