Of the motion of the moving body.

Of the heavy body descending through the air while the elements are in revolving motion, completing a whole revolution in 24 hours.

a b - c d - e f - g o - n m

The moving body descending from the highest part of the sphere of fire will make a straight motion down to the earth, even though the elements were in continuous revolving motion about the centre of the world. Proof: let the heavy body descending through the elements be b, which moves from a in order to descend to the centre of the world m. I say that this heavy body, even though it makes a curved descent in the manner of a helix, will never stray from its rectilinear descent, which runs continuously between the place from which it parted and the centre of the world. For if it set out from point a and descended to b, then in the time it descended to b and was carried to d, the site of a has turned round to c, and so the moving body finds itself upon the straight line that extends between c and the centre of the world. And if the moving body descends from d to f, then c, the beginning of the motion, in the same time moves from c to e; and if f descends to h and turns to g; and thus in twenty-four hours the moving body descends to the earth below the place from which it first parted, and such motion is composite.

If the moving body descends from the highest to the lowest part of the elements in 24 hours, its motion will be composite of straight and curved. I call it straight because it will never stray from the shortest line that extends from the place whence it parted to the centre of the elements; and it will come to rest at the lowest extremity of that straightness, which always lies at the zenith below the place from which the moving body parted. And such motion is in itself curved through all the parts of the line, and consequently is in the end curved along the whole line; and from this it follows that the stone thrown from the tower does not strike the side of that tower before it reaches the ground.