Secular Variations of the Magnetic Needle. 71 



Thus admitting a westerly revolution of the internal fluid mass, 

 results analogous to observed magnetic phenomena would seem 

 of necessity" to follow. 



The question then returns upon us — Does the theory of the 

 earth require, as a necessary physical consequence, a westerly 

 revolution of the internal fused mass ? 



It is a fact noticed by La Place, and indeed one of easy de- 

 monstration, that on the supposition of a gradual cooling process 

 and a consequent diminution of the earth's radius by contraction, 

 the diurnal revolutions of the earth would gradually become 

 more rapid, that is, the length of our day would be gradually 

 diminishing. Every particle of a revolving sphere, on falling 

 towards the axis by a general contraction, tends, by preserving 

 its absolute velocity, to a more rapid angular motion, and the pe- 

 riod of revolution for the whole mass must be inevitably dimin- 

 ishing. In the case of the earth, it is true that astronomy has 

 not detected any change in the length of our day — a fact by no 

 means incompatible with the existence of such a change ; for in 

 the first place the extreme accuracy which now marks astronom- 

 ical observations is comparatively of modern date ; and secondly 

 it is stated as the result of calculation that a contraction of the 

 radius of the internal fused mass of one twenty fifth of an inch 

 in a century would be sufficient to account for all the results of 

 volcanic action at the present age of the world. That astronomy 

 has been unable to detect the minute acceleration of the diurnal 

 motion which has accrued since men began to converse with the 

 stars on the plains of Chaldea, constitutes therefore no valid ob- 

 jection to the truth of a geological theory involving such a result. 

 Improved methods, exact and long continued observations, will 

 doubtless make astronomy a competent witness on this point in 

 time to come. If the theory of the earth be true, it must be ad- 

 mitted as a necessary physical consequence, that the length of 

 the day is diminishing ; though the decrement at this age of the 

 world may be inappreciable from century to century, and the 

 records of astronomical science may as yet furnish no evidence 

 of the fact of such diminution. 



But this tendency to increased angular motion arising from 

 contraction, must obviously be greatest in those portions of the 

 mass of the earth which have contracted most, and least in those 

 parts which have contracted least. Now the earth having been, 



