In the second world war a new type of bomb was invented by Barnes Wallis for use against dams. The bomb was launched from a low-flying aircraft on to the surface of the dammed water in the direction of the retaining wall. For a sufficiently shallow angle of incidence the bomb bounced along the surface of the water, like a skimming stone, until it struck the dam wall. The bomb then sank to the base of the wall and was exploded by a depth sensitive detonator. Navel gunners of the 17th and 18th centuries had used this effect to increase the range of their guns. Projectiles will bounce off water only if the angle of incidence with the water is smaller than a certain value.