ASTRONOMY. 
217 
as, had blind chance, had a casual impulse, had a stroke or 
push at random, set the earth a-spinning, the odds were in¬ 
finite, but that they had sent it round upon a wrong axis. 
And what would have been the consequence? The differ¬ 
ence between a permanent axis and another axis is this. 
When a spheroid in a state of rotatory motion gets upon a per¬ 
manent axis, it keeps there; it remains steady and faithful to 
its position; its poles preserve their direction with respect 
to the plane and to the centre of its orbit: but whilst it 
turns upon an axis which is not permanent, (and the num¬ 
ber of those we have seen infinitely exceeds the number 
of the other,) it is always liable to shift and vacillate from 
one axis to another, with a corresponding change in the 
inclination of its poles. Therefore, if a planet once set off* 
revolving upon any other than its shortest, or one of its long¬ 
est axis, the poles on its surface would keep perpetually 
changing, and it never would attain a permanent axis of 
rotation. The effect of this unfixedness and instability 
would be, that the equatorial parts of the earth might be¬ 
come the polar, or the polar the equatorial; to the utter de¬ 
struction of plants and animals, which are not capable of 
interchanging their situations, but are respectively adapted 
to their own. As to ourselves, instead of rejoicing in our 
temperate zone, and annually preparing for the moderate 
vicissitude, or rather the agreeable succession of seasons 
which we experience and expect, we might come to be 
locked up in the ice and darkness of the arctic circle, with 
bodies neither inured to its [rigors, nor provided with shel¬ 
ter or defence against them. Nor would it be much bet¬ 
ter, if the trepidation of our pole, taking an opposite course, 
should place us under the heats of a vertical sun. But if 
it would fare so ill with the human inhabitant, who can 
live under greater varieties of latitude than any other ani¬ 
mal; still more noxious would this translation of climate 
have proved to life in the rest of the creation; and most 
perhaps of all, in plants. The habitable earth, and its 
beautiful variety, might have been destroyed by a simple 
mischance in the axis of rotation.* 
* The earth being an oblate spheroid, we may suppose it to be cut by 
a plane passing through A B, Fig. 3, Plate XXXIX, which may represent 
its axis, and the common section of this plane with the spheroid will be 
an ellipse like A D B E ; of this ellipse A B will be an axis ; and, from 
the property of the curve, it will also be the shortest line which can 
be drawn through the centre C. If now the diameter D E be drawn at 
right angles to A B, it will be the longest line which can be drawn in the 
ellipse, and it will represent a diameter of the equator. As the plane 
T 
