_Total Weight per Horsepower Hour._ As is the case in almost every point in the design of the dirigible, conflicting conditions must be reconciled in order to provide it with a power plant affording sufficient speed with ample radius of action. It has already been pointed out that power requirements increase as the _cube of the speed_, making a tremendous addition necessary to the amount of power to obtain a disproportionately small increase in velocity. In this connection there is a phase of the motor question that has not received the attention it merits up to the present time. The struggle to reduce weight to the attainable minimum has made weight per horsepower apparently the paramount consideration—a factor to which other things could be sacrificed. And this is quite as true of aeroplane motors as those designed for use in the dirigible. But it is quite as important to make the machine go as it is to make it rise in the air, so that the question of _total weight per horsepower hour_ has led to the abandonment of extremely light engines requiring a great deal of fuel. Speed is quite as costly in an airship as it is in an Atlantic liner. To double it, the motor power must be multiplied by 8, and the machine must carry 8 times as much fuel. But by cutting the power in half, the speed is reduced only one-fifth. The problem of long voyages in the dirigible is, accordingly, how to reconcile best the minimum speed which will enable it to make way effectively against the prevailing winds, with the reduction in power necessary to cut the fuel consumption down to a point that will insure a long period of running. When the speed of the dirigible is greater than that of the prevailing wind, it may travel in any direction; when it is considerably less, it can travel only with the wind; when it is equal to the speed of the latter, it may travel at an angle with the wind—in other words, tack, as a ship does, utilizing the pressure of the contrary wind to force the ship against it. But as the air does not offer to the hull of the airship, the same hold that water does to that of the seagoing ship, the amount of leeway or drift in such a manoeuver is excessive. This applies quite as much to the aeroplane as it does to the dirigible. FRENCH DIRIGIBLES