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[I]

The middle of the straight [portion of the] path made by a weighty body hurtling through the air with violent motion1 is of greater power and greater percussion against what opposes it than any other part of that.2

The reason for this is that, when the weight departs from the force of its motor, although this departure is at the first degree of its power, (nonetheless, finding the air without motion), the weight finds the air at the first degree of resistance.  (And although the air has a greater sum of resistance than the power of the weight being thrust into it), nonetheless, in striking only a small part of the air, the weight comes to finish as the victor).  So the weight drives the air from its site and, in driving the air, it impedes3 its own velocity somewhat.  (Therefore, as this air is pushed, that pushes and drives other air, generating circular4 movements behind itself, and for these circles the weight moved therein is always the center, just as for circles made in the water, which have the place struck by the stone as their center.  And so, because one circle drives another, the air that is in front of its motor — all along that line — is prepared for movement, and this effect increases the more, the more the weight driving the air presses the air.  So this weight finds less resistance in the air and redoubles its course, with more velocity, like a boat being drawn through the water, which is moved with difficulty in its beginning motion, even though its motor is at its most potent strength.5  And when this water begins to take motion, with arched waves, the boat finds little resistance in following that motion, so it moves with greater facility.  Similarly,6 the shot, finding little resistance, follows the beginning course until the point where its first strength has abandoned it somewhat, and it starts to weaken and decline.  Then, as it changes course, the escape prepared before it by the fleeing air is no longer of any use,7 and the more it declines, the more the shot finds increasing8 resistance in the air, and so the more it slows down, until such time when, resuming its natural motion, it regains more velocity.  Now I conclude9, by the reason of the eighth proposition,10 that the part of the motion found between the initial resistance of the air and the beginning of its decline is of the greatest power, and this is the middle portion of the path, which is made through the air in a straight and direct line.

When a boat twists, it, also, slows in its course.

And because no place can be empty, the place where the boat starts from tends to close again, … 4 … and, indeed, this makes a force11 that presses and thrusts the boat.

4 … like when the stone12 of a cherry is squeezed by the fingers …