Although the dirigible balloon, a Zeppelin, for example, may hover in a stationary position at will when dropping bombs, still it constitutes such an enormous target that it must fly very high in order to keep out of range of gun-fire. Guns are now made which can reach air-craft at the height of two miles. At that height, or at half that height, there can be but little accuracy in bomb-dropping, even from the stationary Zeppelin. The efficiency of a fighting machine is exactly proportionate to the amount of life and property that it can destroy in a given time with the minimum exposure of property and life in order to do the work. If a fleet of a dozen Zeppelins should be able to attack and destroy an entire British fortified town like Dover, it would be a good investment. If, however, the loss that it would be able to inflict upon the enemy were only equal to the loss that the British would inflict upon it, then it would be a bad investment, or at least, an investment without profit, for the reason that, in war, it is poor policy to risk the destruction of a valuable war-engine merely for the destruction of what may be termed non-belligerent property of an enemy, such as the dwellings of the inhabitants of a city. Suppose, for example, that a couple of Zeppelins should be able to destroy houses in a British town having a value ten times as great as the value of one of the Zeppelins, and, in the attack, should lose one of the Zeppelins, it would not be a profitable raid, for a Zeppelin, being useful for scouting purposes, is a potential factor in deciding the issue of the war, whereas the houses have practically no bearing on the issue of the war. It is good policy to use both men and machinery of war only for the destruction of men and machinery of an enemy, and not for the destruction of non-combatant inhabitants and property. Much has been said about gun-fire from air-craft upon underlying troops. A man standing on the earth, being seen endwise, presents a much smaller target to the vertical fire of the air-man than he presents when fired at horizontally from the earth, because in the one case he is seen end-to, and in the other case side-to. Besides, several other men may be exposed to the horizontal fire. The air-man, however, is a conspicuous target, and if his machine is hit and crippled the result is fatal to him. AËROPLANE AND DIRIGIBLE COMPARED As I have for many years predicted, the chief use of air-craft, whether aëroplane or dirigible balloon, is for purposes of reconnaissance. This war has amply demonstrated the fact that air-craft are of enormous value. They have rendered surprises in force practically impossible. Each side has been able to keep itself fully aware of the numbers and disposition of opposing troops. The aëroplane costs but a fraction of what the Zeppelin costs, while the Zeppelin presents a target enormously larger. It constitutes a target so big as to make the broad side of a barn blush with envy.