brought into the field by the strategic plan, but rather such portion of it as can be brought to bear simultaneously on the enemy; it will become a battle of airmanship. One of the difficulties which exists to-day, and probably will always be a matter of anxiety, is that of signalling; and to whatever extent this remains a difficulty, the flexibility of the formation will be impaired. The air fleet with the most perfect system of signalling will be the best able to take advantage of any opportunity that may arise in the course of an engagement by adapting its tactical scheme to the needs of the moment. In order to render it possible to control large numbers, it would appear to be evident that the unit command will be a small group, or what is at present termed a "flight" (some four or five strong), and that the individual machines will act on the plan of follow-my-leader; in other words, they will have instructions to keep station. It may be found desirable to extend the same system to larger units, as tending to avoid possible confusion.
§ 97. The "V" formation and its value. There is a point in connection with the pattern or character of these unit formations which may turn out to be of importance. It has for long been observed that certain birds, flying numbers strong (as in migration), are in the habit of assuming definite formation, and that this formation is of the shape of a letter V travelling point first; each bird, besides being some distance behind its leader, is also somewhat on one or the other flank. The reason for this is almost certainly one of aerodynamics; the air immediately in the wake of a bird in flight has residuary downward motion, and so is "bad" air from the point of view of the bird following. On the other hand, the air to the right and left of the leader has residuary upward motion owing to the vortical character of the wake disturbance, and so is "good" air; con-
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