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On The Heavens   

earth has weight until it reaches the centre, and the jostling of

parts greater and smaller would bring about not a waved surface, but

rather compression and convergence of part and part until the centre

is reached. The process should be conceived by supposing the earth

to come into being in the way that some of the natural philosophers

describe. Only they attribute the downward movement to constraint, and

it is better to keep to the truth and say that the reason of this

motion is that a thing which possesses weight is naturally endowed

with a centripetal movement. When the mixture, then, was merely

potential, the things that were separated off moved similarly from

every side towards the centre. Whether the parts which came together

at the centre were distributed at the extremities evenly, or in some

other way, makes no difference. If, on the one hand, there were a

similar movement from each quarter of the extremity to the single

centre, it is obvious that the resulting mass would be similar on

every side. For if an equal amount is added on every side the

extremity of the mass will be everywhere equidistant from its

centre, i.e. the figure will be spherical. But neither will it in

any way affect the argument if there is not a similar accession of

concurrent fragments from every side. For the greater quantity,

finding a lesser in front of it, must necessarily drive it on, both

having an impulse whose goal is the centre, and the greater weight

driving the lesser forward till this goal is reached. In this we

have also the solution of a possible difficulty. The earth, it might

be argued, is at the centre and spherical in shape: if, then, a weight

many times that of the earth were added to one hemisphere, the

centre of the earth and of the whole will no longer be coincident.

So that either the earth will not stay still at the centre, or if it

does, it will be at rest without having its centre at the place to

which it is still its nature to move. Such is the difficulty. A

short consideration will give us an easy answer, if we first give

precision to our postulate that any body endowed with weight, of

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