12 
19. Bat while this translation theory of Poisson is both 
vague and inadequate, and wanting in direct evidence, the fact 
of the movement of our system in space is a strong reason 
against the uniformity assumed by many geologists to have 
lasted through many millions of years. The rate of the sun’s 
motion in space is held to be 150 millions of miles a year. 
This would carry it as far as a Centauri, the nearest star whose 
parallax is determined, in 140,000 years. The direction pro- 
longed backward has its apex only 25° from Sirius, the 
brightest of all the stars, and of which the light has been 
reckoned to be 60 times greater than that of the sun. Its 
parallax is of a second. It has been lately inferred from 
the spectroscope that we are receding from Sirius at the rate 
of 25 miles a second, or 800 millions in a year, so as to 
traverse the whole distance in 100,000 years. And since we 
cannot tell whether the earlier motion may not have valued so 
far in its direction, we can have no assurance that all the 
elements of our system may not have been altered by the 
proximity of Sirius only one hundred thousand years ago. All 
estimates of solar force and the earth’s inclination and ex- 
centricity which go back beyond this limit must remain highly 
uncertain on this ground alone, and are beyond the range of 
assured and certain science. 
20. Two other theories may be also dismissed in few words. 
First, that of an altered axis of rotation, so that the north and 
south poles of the diurnal rotation were at places considerably 
remote from those which they now occupy. But this is 
rendered all but impossible by the spheroidal shape of the 
earth. At any time, after the crust had once hardened and 
taken a spheroidal form, revolution on any axis, not 
adjacent to the present one, must have been mechanically 
impossible. Any secondary change of surface by the uprising 
of a mountain-chain might produce an increased nutation and 
a kind of waddling motion around the true axis, but it could 
not alter the place of that axis, or produce any sensible effect 
on the climate of any main parts of the surface. 
21. Another theory of the same kind is Sir C. Lyell’s 
transposition theory. He supposes that the mean tempera- 
ture would be raised if the land were mainly in the torrid 
zone, and be lowered if it were grouped around the poles. 
Mr. Croll argues that the effect would bo diametrically 
opposite, and that the contour of the surface most favourable 
to tho warmth of tho earth is when the water is in all the 
middle part, and tho land only at the poles. Now it is difficult 
to reason out all the consequences as to the mean temperature 
of the whole surface. The mere fact that two such opposite 
