A rocket shot with a crossbow. Of the galley trumpet. The galley trumpet pushes backward the one who holds it in his hand, because the flame that multiplies within it wants to extend itself in contrary directions.

It is proved by the motion of the rocket, which with part of the flame that extends within it pushes that rocket backward, and with the opposite part drives away the air that condenses before it. It is proved thus: the powder that catches fire grows between the spent powder that closes it behind and the air that condenses before it, and with both extremes it pushes against the aforesaid objects, thrusting away from itself with greater distance the one that offers less resistance, and so on. It is doubted which part of the flame condenses most in the trumpet where it is generated, because on one side it is generated at the border of the powder and on the other it condenses at the border of the air, together with that air. And here it seems one must judge that within one and the same fluid there cannot exist side by side various rarities or densities, because the excess supplies the deficiency and the deficiency the excess, and so the non-uniformity of such a fluid makes itself uniform. It is answered that the flames of two joined lights have greater density at their borders than at their outer extremes; but this arises because the one flame exerts no force against the other, both running toward one and the same quarter, and so on.

Where the flame condenses most in the trumpet. When two [powers] flee in contrary directions and dilate a rarefiable body, that part of the body will become the rarest which is nearest the middle of its length. This we see in the cylinder made of warm wax and pulled from its opposite ends in contrary motions.

When two powers concur with equal forces one against the other in the pressing of a condensable body, that part of the constrained body will become the densest which is nearest the middle, or rather most remote from the opposite ends, and so on.