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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 — consequence, a westerly 
revolution of the internal fused mas 
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, 
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