The mechanics of gravity, and it's effect on flying objects, was still over a century in the future when cannon began to become common When gravity was defined in the 17th century, "ballistics" promptly became an area of intense study. Artillerists have, ever since, been carefully studying all the elements that can effect the projectiles they fire. It is now known that, in addition to gravity and accelleration, other items such as wind, air density, humidity and the shape of the fired object all play a signifigant role. In contrast, during the Hundred Years War, only long experience with the guns could produce accurate gunners, who kept their knowledge a closely guarded secret. They probably knew something about the curving trajectory which a cannon ball describes, but didn't bother telling anyone. The conventional wisdom was that the ball travelled in a straight line until it ran out of energy, and then promptly dropped to the ground. Despite this, artillery became increasingly effective during the wars.