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Rh safe at an altitude of somewhat over 7000 ft.; it is in that region that the top of the trajectory lies for vertical shooting.

The duties of a strategic scout on long-distance work would, without doubt, permit of flying at such a high altitude, and it may be added that, although absolute immunity is not reached at less than about 7000 ft., a solitary aeroplane can only present a very unprofitable target at far lower altitudes. In fact, it may be taken that at, say, 5000 ft, or 6000 ft., the amount of small-arm ammunition required to bring down an aeroplane would be enormous. Not only has the velocity become so reduced as to render a "hit" capable of but little mischief, but the time of flight of the bullet, rising vertically to this altitude, would be about 8 or 9 seconds and the distance moved by the aeroplane 1000 ft., more or less. Therefore it would be necessary to fire into quite a different part of the heavens from that in which the aeroplane is seen, something akin to sighting into the Great Bear to hit the Pole Star. Beyond this the gyroscopic drift of a bullet fired vertically is nil, against some 30 ft, or 40 ft, under normal conditions; also the error due to the earth's rotation is a matter of about 30 ft, westward, and cannot be allowed for without taking reference to the compass bearing. Taking all these things into account, it is evident that for the infantryman or gunner not specially trained, the task of bringing down an aeroplane flying at high altitude is no light one, especially when we recall the fact that for every inclination and bearing of the line of sight, the conditions differ. In designing the mounting of aeroplane-stopping artillery or machine-guns, it would be possible to render the sighting corrections for such items as gyroscopic