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§ 3. Aeroplane and Dirigible: Speed Limitations. Two questions are involved in the consideration of the relative merits of the aeroplane and dirigible. We are firstly concerned with their respective advantages and disadvantages in relation to their primary function—namely, as instruments of reconnaissance, attack, and defence; secondly, we have to take into account their secondary function—i.e. their relative power of mutual destruction; the question whether, for example, either can drive the other from the field, or whether each may have its own rôle to play in securing and holding the command of the air.

Before going into either of these questions in detail it is convenient to review a few of the facts by which limitations are imposed on the ultimate performance of either type of aircraft. We must avoid falling into error by judging each too closely by its performance of to-day.

The all-important question of speed is a matter depending primarily on the lightness {i.e. horse-power per given weight) of the prime mover, and the law of resistance. The horse-power per unit weight of motor is roughly the same whichever type of aircraft is in question, and any future advance in the art of motor construction tending to diminish weight will, we may presume, be equally available for either type. The laws of resistance of the aeroplane and dirigible are