Gravity and levity are equal powers between themselves, the quantities being equal and born near one another.

Given the cause, nature works the effect in the shortest way that can be done.

No element is heavy or light in its natural place.

Gravity and levity are caused by one element being drawn into another.

Equal quantities of naturally contingent elements, being exchanged of place, resist one another equally.

First principles of the heavy and the light.

The body of the lightest elements.

Spherical is the figure of the lightest elements; and nothing is smaller or lower than its centre, and nothing is higher than that which is most remote from that centre.

Infinite are the diameters of every spherical body.

Those bodies of equal quantity will be said to be one beneath the other whose centres of gravity are nearer to the centre of the elements.

No line that extends from the centre to the circumference is other than perpendicular and straight.

No straight line that passes through the sphere, without touching its centre, will be other than oblique.

More oblique is that straight line which passes through the spherical body farther from its centre.

Less oblique is that line which passes through the sphere nearer to that centre.

a b is the aperture through which the sun passes. And if you could measure the thickness of the solar rays at n m, you could rightly set the true lines of the concourse of those solar rays, with the mirror standing at a b, and then make the reflected rays between equal angles toward n m. But since you cannot take them at n m, take them within the aperture at c d, where they can be measured at the striking of the solar ray, and then place your mirror at the distance a b and there let the rays d b, c a fall, and then let them rebound between equal angles toward c d. And this is the true way; but you must operate this mirror in the same month and same day and hour and instant, and it will do better than at any other time, because at such a distance of the sun such a pyramid of rays was caused.

It is possible for a heavy thing of itself to stand upon a light one.

When the lightest elements of two worlds were contingent, the heavy body whose centre lay in such contact would be without motion.

The weight: why does it not remain in its place? It does not remain because it has no resistance. And whither will it move? It will move toward the centre. And why not along other lines? Because the weight, having no resistance, will descend downward by the shortest way, and the lowest place is the centre of the world. And how does such a weight know to find it with such brevity? Because it does not go, like something insensible, first wandering along diverse lines.

A drum of notches rubbed by wheels of springs.

A square drum whose skin is tightened and slackened with the lever a b.

A drum in consonance.

A board in consonance, that is, three boards together.

Just as one and the same drum makes low and high voices according to the skins being more or less tensioned, so these skins, variously tensioned over one and the same drum body, will make various voices.

Keys that are narrow, and they close holes at great distances between them, and they serve the purpose of the trumpet next above at a b.

Let a enter in place of the ordinary stops that practitioners have in the holes of their fifes.