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Rh job by firing the wreckage remaining after high explosives have done their work.

§ 65. The Bomb: Di^culties in Connection with Aiming. The accurate directing or aiming of bombs or hand-grenades, or of any gravitationally-propelled missile, is one of great difficulty, and many suggestions for the improvement of the degree of precision attainable have been made. The problem, in a sense, is the inverse of that of firing at an aeroplane at high altitude. The period during which the projectile is at the top of its trajectory (the beginning of its fall in the case of the bomb), and in which its velocity is low, introduces considerable uncertainty as to direction; it has been proposed to minimise this difficulty by giving the bomb an initial velocity or "send off," by some form of spring or pneumatic gun. The factor affecting the aim definitely known to the pilot is the velocity of flight (relatively to the air); the factors less exactly known are the height, the direction and velocity of the wind, or, as it must be reckoned by the aeronaut, the earth drift, and the direction of the vertical. Previous observation may have given the approximate wind allowance, and the barometric reading (the aneroid) will give the altitude, which, in conjunction with a contour map, will give the pilot the figure for his height. The determination of the vertical, or "plumb," is far less simple or certain than may at first sight appear, since any pendulum device is affected by acceleration just as much as by gravity, and the reading of a damped pendulum or a spirit-level gives the apparent plumb, which may be literally anywhere.

In the case of a machine "looping the loop," for example, the apparent plumb is, in fact, at one instant diametrically opposite to the true plumb, and during the whole evolution it "boxes the compass" in a vertical