(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
ADVANTAGES AND DISADVANTAGES OF AIRSHIPS
The chief advantages of aircraft that are lighter than air over those that are heavier than air in warfare are:—
1. Their speed can be variable.