the fastest dirigible is capable. There is every prospect that its advantage in this respect will increase rather than diminish with the march of progress.
Beyond the above, it is well understood that an increase in size is conducive to a reduction in the resistance coefficient; this applies to both aeroplane and dirigible. This fact has been one of the controlling considerations in dirigible design; no dirigible, other than of comparatively large size, has been found to be of real service. It is, moreover, evident that, in the case of some of the large Zeppelins, it will not be found practicable to go very much further in the direction of increase. Here again the aeroplane is at an advantage; we can in nowise regard the aeroplane of to-day as defining the limit.
It is abundantly manifest therefore that the dirigible is at a permanent disadvantage of not less than two to one in the matter of speed.
§ 4. Aeroplane and Dirigible: other points of comparison. The question of range and duration of flight is largely determined by petrol-carrying capacity. In the aeroplane both range and duration depend definitely upon the petrol supply holding out; in the case of the dirgible the same applies to a limited extent; but here the duration and, to a less extent, the distance can be greatly prolonged by reducing the speed to the minimum possible without jeopardising the control. In the dirigible the gradual loss of buoyancy, due to the leakage and escape of hydrogen, is an independent determining factor. Taking everything into account there is not much to choose between the two types of aircraft in the matter of range or radius of action; on the other hand, under favourable conditions, the dirigible has undoubtedly the advantage on the score of duration of flight. The maximum is about 24 hours in
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