The theory of projectiles moving under gravity is well known. The parabolic trajectories can be calculated and their envelope, as the direction but not the magnitude (u) of the initial velocity is varied, is the curve of safety, or bounding parabola. Where a given trajectory meets the envelope, the particle is moving at right-angles to its initial direction. In particular, the maximum range on a horizontal plane is u2/g and is attained by projection at 45° above the horizontal.