Others thought that a shot, on leaving the muzzle, described a straight line; that after a certain period its motion grew slower, and then that it described a curve, caused by the forces of projection and gravity; finally, that it fell perpendicularly. Tartaglia seems to have originated the notion that the part of the curve which joined the oblique line to the perpendicular, was the arc of a circle tangent to one and the other.
Galileo, 1638.
In the year 1638, Galileo, also an Italian, printed his dialogues, in which he was the first to describe the real effect of gravity on falling bodies; on these principles he determined, that the flight of a cannon shot, or of any other projectile, would be in the curve of a parabola, unless it was deviated from this track by the resistance of the air. A parabola is a figure formed by cutting a cone, with a plain parallel to the side of the cone.
GRAVITY.
Bullet as influenced by powder and gravity only.
We will now proceed to consider the course of a bullet, as affected by two forces only, viz., 1st. The velocity communicated to it by the explosion of the powder; and 2nd. By the force of Gravity.
The attraction of the earth acts on all bodies in proportion to their quantities of matter.
If no air, all bodies would fall in same time.
The difference of time observable in the fall of bodies through the air, is due to the resistance of that medium, whence we may fairly conclude, that if the air was altogether absent, and no other resisting medium occupied its place, all bodies of whatever size, and of whatever weight, must descend with the same speed. Under such circumstances, a balloon and the smoke of the fire would descend, instead of ascending as they do, by the pressure of the air, which, bulk for bulk, is heavier than themselves. Gold and dry leaf in same time.A dry leaf falls very slowly, and a piece of gold very rapidly, but if the gold be beaten into a thin leaf, the time of its descent is greatly prolonged. If a piece of metal and a feather are let fall at the same instant from the top of a tall exhausted receiver, it will be found that these two bodies, so dissimilar in weight, will strike the table of the air-pump, on which the receiver stands, at the same instant. Supposing the air did not offer any resistance to the onward course of a projectile, and that the instantaneous force communicated to a bullet, from the explosion of the gunpowder, were to project it in the line A.B. ([plate 21], fig. 4.) from the point A., with a velocity that will send it in the first second of time as far as C., then if there were no other force to affect it, it would continue to move in the same direction B., and with the same velocity, and at the next second it would have passed over another space, C.D., equal to A.C., so that in the third second it would have reached E., keeping constantly in the same straight line.