Page:Aircraft in Warfare (1916).djvu/140

§ 65 plane. The lateral deviations of the apparent from the true plumb, are no less serious, and, whenever a machine is turning and correctly banked, the spirit-level records the machine as being on an even keel. In other words, an error in the reading under these conditions is equal to the angle of banking, and is quite commonly as much as 30 deg, or 40 deg. It is precisely on this point—the confusion of the true with the apparent plumb—that many of the suggestions offered for the direction of bomb-dropping are found to fail; and it is quite useless for those having no knowledge of the principles involved to attempt to deal with the problem. No better way of obtaining a clear conception of the difficulty exists than a study of the pendulum accelerometer.

In the case of a modern aeroplane which virtually "flies itself" it is possible to determine the true plumb with considerable exactitude under calm atmospheric conditions. Such a machine will, just like a gliding model, settle down to a definite flight velocity, known as its natural velocity, and to a known gliding angle, and will maintain a tolerably straight path; under these circumstances the apparent plumb is the true plumb. When, however, atmospheric disturbances are present, the difficulty once more makes it appearance.

In view of the above, it is doubtful whether bomb-dropping from aeroplanes will ever be found to compete with gun-fire on the score of accuracy, and it may be anticipated that the utility of this mode of offence will be confined mainly to attack on positions or objects that present a mark either of sufficient area or size to be easily hit, or of sufficient importance to justify a disproportionate expenditure of missiles.