We shall see presently that the force of the wind can be utilized to a certain extent to make up these losses, but still some muscular effort should be required. If our vulture or albatross would only occasionally deign to flap a wing, all would be well. His obstinacy is very perplexing.
Leaving the birds to their own peculiar devices, let us now consider what principles should guide us in constructing a flying machine.
In the first place, by acting on the air, the machine should be able to lift itself from the ground; and, leaving out of account small models, this is a preliminary no one appears so far to have succeeded in. Many pictures may be seen of flying-machines booming along through the air with all sails set, passengers evidently happy, some serenely smoking, others promenading the deck in the usual way, with perhaps a couple behind the wheel-house; but a representation of a machine just on the point of starting out is not to be met with.
In order to produce an upward pressure or reaction, the wings or propeller acting on the air evidently should drive it downward. Suppose now that our machine weighs 600 pounds, and that it has the same propelling surface in proportion to its weight as the Australian crane, we should then need about 246 square feet, and a pressure of 2·4 pounds acting upward on each square foot would lift it from the ground.
Referring again to the table giving the relation between wind velocity and pressure, we notice that a pressure of 2·4 pounds would be occasioned by a velocity of about twenty-two miles an hour.
If, then, we should cause our propeller—be it a screw or wings, or any other form—to drive downward a current of air at this rate, the cross-section or area of the current being 246 square feet, the total upward reaction would be great enough to raise the machine.
Of course, for any other proportion of wing-surface to weight, our table would give other results; or if the air is already in motion, it will tell us what increase of velocity should be given to produce the desired pressure.
The results given in the table can also be readily found in a purely theoretical way, and they seem so important that it is a wonder investigators have given them little or no attention.
A machine possessing weight can fly only by doing something to the air. It must put the air in motion, and it can be shown that the amount of this motion will be a measure of the work done and reaction obtained.
If air is already in motion, we can not utilize its force, not wishing to drift along, except by changing in some way its velocity.
Granting all this, our table or formula will tell us, not only what volume of air must be used to gain the desired reaction or motion, but also the least power necessary. Knowing the weight of and ve-
