(_a_) _The height of the object._--This may be accurately determined from the contour lines on the map, or from a determination of its normal barometric height. Both must be done before starting. (_b_) _The height of the airship above the object._--The barometric height is read and reduced to normal conditions. The difference in heights as found from (b) and (a) gives the height above the object. (_c_) _The velocity of the wind._--May be read on an anemometer in the airship, or determined beforehand by captive balloons. (_d_) _The time of fall._--Given by the law of gravitation from the determination under (b). The height of fall = h = gt²/2. Whence the time of fall t = √(2h/g). (_e_) _The resistance of the air._ R = (γ/g)Fv². (_f_) _The leeway._--The longer the fall, and the lighter and larger the falling body, the stronger is the drift. For known missiles, the drift for different heights and wind velocities may be determined practically. (_g_) _Unsteadiness of the airship._--The irregularity of the pressure of the wind, and its constant variation in direction, renders it impossible for the airship to remain perfectly steady. The elements stated under (_b_) and (_f_) must be rapidly determined, and suitable tables have been prepared for this purpose. The irregularity of the wind and the peculiarities of the airship mentioned under (_g_) render a preliminary trial necessary. The drift also is determined by this method, before the large air-torpedo is cast out. The air-torpedo must be brought by sight vertically over the object by steering the airship, the value of the mean drift previously determined being allowed for. In throwing out a missile while actually travelling, the velocity of the airship must be taken into account, as well as the elements (_a_) to (_g_) given above, since this velocity is also possessed by the body thrown out. The determination of the proper point is now greatly increased in difficulty. Its position is a function of the relative height of the airship above the object, of the velocity, and of the drift, and allowance must be made for all these factors. For this purpose, motion, either with or against the wind, is the simplest. On account of the point on the earth over which the missile must be thrown out not being in general well marked, it is necessary to use also angles of sight. The problem before the aëronaut is, then, as follows:--For a given height, velocity, and drift to find the necessary angle of depression at which the missile must be thrown out in order that it may fall on to the object. The casting out of the missile against the object while travelling is governed, therefore, by the same rules as those governing the discharge of a torpedo from a torpedo-boat. CHAPTER V