We see that the gnat, one of the lightest of insects, has an expanse of wing of no less than 489 square feet for each pound of weight, while the heavy cockchafer has only 5.1 square feet for each pound. With birds, the sparrow has 27 square feet of wing-surface for each pound of weight, while the great Australian crane has only 0.41 of a square foot, and yet this bird undertakes remote journeys, and, the eagle excepted, flies higher, and keeps on the wing longest, of all the travelers.

It would appear, then, that our flying-machine, while heavy, need not necessarily have a very broad expanse of flying surface. Indeed, paradoxical as it may seem, weight is really an essential feature. Set in motion by muscular effort, the weight of a bird acts somewhat like the fly-wheel of an engine: the power is stored up during the downward stroke of the wing, to be given out again on its upward stroke, and probably it is weight also that enables the bird to successfully combat and take advantage of the force of the wind. It is noteworthy that all sailing-birds, like the hawk or vulture, have comparatively heavy bodies. The magnificent albatross, in rising from the water, is said to beat the air with great energy, but, when fairly launched, in a brisk gale, will sweep around in broad circles for hours together, hardly ever deigning to flap a wing. Darwin, in his "Voyage of the Beagle," speaks of watching the condor sailing in a similar way at a great height, without, so far as he could notice, any flapping action whatever.

At the same time, it is hard to understand how such a condition of affairs could exist. The condor's wings, inclined to the wind, have been compared to a kite, and if there were a string stretching from the bird to some fixed point, the whole thing would be clear; but every boy knows to his cost that, if the string slips or breaks, the kite quickly seeks some other point of support-probably a telegraph-wire. But Professor Pettigrew has suggested that the string is the invisible one representing the attraction of gravitation, and that "the string and the hand are to the kite what the weight of the flying creature is to the inclined planes formed by its wings." This, however, does not make the matter much clearer, for the force of gravity acts in vertical lines, and a vertical kite-string, with the kite flying directly overhead, is a thing, it is safe to say, no boy ever saw. Why should not our bird drift with the wind unless he uses some muscular effort to overcome its force or to keep himself from falling?

Once elevated, he can utilize his weight in a number of ways. A body will naturally fall along a line of least resistance, and if the front edge of the wings be tipped slightly downward the bird will glide forward while falling, gaining velocity and momentum ; and then, by reversing the inclination of the wings, he can again glide up an aërial incline until this stored-up energy has been expended. But the resistance of the air must be overcome, and there must be continual loss from the imperfect sustaining power of the wings.

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 24 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

locity impressed upon the air, downward or in any other direction, it becomes an easy matter to determine the power.

For example, in the practical case just considered, to lift the machine from the ground would require an expenditure of at least eighteen horse-power. This is the least power that would do the work -the actual power would depend entirely upon the efficiency of the propeller.

Having at last succeeded in getting away from the ground, we wish to fly in any direction-to set the birds an example of how the thing ought really to be done.

Here, again, we must apply the principles just announced. To go forward, the air must be driven aft. Knowing the speed proposed, our table will give us at once the resistance for each square foot; and knowing the size or bulk of our machine, we can readily estimate the power required.

The management of the wind unquestionably will be a very important factor in the construction of a flying-machine; indeed, it may be considered the most troublesome part of all. Properly handled, the wind might be made a useful servant, otherwise a dangerous

master.

The only plan that suggests itself is through the use of an inclined plane. Here, at any rate, we must imitate the birds.

My attention was not long ago called to an article on Aëronautics, in the Proceedings of the New Zealand Institute for 1878, and in it was a table from experiments by Mr. Skye, giving the lifting power of the wind, blowing at the rate of twenty-three miles an hour upon a plane surface, one square foot in area, inclined at various angles. These figures lead to some very surprising and interesting results :

Angle plane makes with wind.

Lifting force, in pounds. Drifting force, in pounds. Ratio between the two.

It will be seen from the second column that while the greatest lifting effect occurs at about an angle of 40°, even at so small an angle as 5° it is still considerable. The third column gives values for the corresponding horizontal pressures; that is, the force which tends to move the plane in the direction of the wind. The fourth column gives the ratio between the two.

It will be seen that the drifting force diminishes at a much faster rate than the lifting force, as the angle of inclination of the plane becomes less.

Consider again the flying-machine weighing 600 pounds, and sup

pose that, in addition to the propeller, we furnish it with an inclined plane having the same area, or, perhaps after the manner of birds, make the propeller act also as an inclined plane; and let it be inclined five degrees, with the wind blowing at the rate of twenty-three miles an hour. Then the table shows us that the total lifting force due to the wind would be 278 pounds, leaving 322 pounds to be supported in some other way. The horizontal or drifting force would be 0.23 pounds on each square foot, or only 56 pounds altogether. To counteract this, let us make our propeller act as a kite-string by sending backward the air at an increased velocity. Our other table tells us how great this velocity should be, and makes the necessary power amount to only about half a horse-power. To support the balance of the weight, we should need also to send downward a current of air, involving an additional expenditure of about seven horse-power.

Combining the two, we get this extraordinary result, that while nearly nineteen horse-power was necessary to lift our machine from the ground, it could hold its own in a breeze of twenty-three miles an hour with an expenditure of only seven and a half horse-power.

No account has been taken of the wind blowing against dead surfaces, such as the body of the bird or machine. This, of course, would depend upon the shape. A bird's body is long and narrow, cleaving the air without great resistance, and a flying-machine should be fashioned similarly.

Other losses have not been considered, but still the broad result holds that it is possible in this way to utilize part of the energy stored up in the wind. The accuracy of the results will depend upon that of Mr. Skye's table; but if future experiment should verify it, we can understand why it is that the albatross, and wild-duck, and heavy birds generally, while rising with great difficulty, when once up keep on the wing with so much apparent ease.

However, there is still the necessity for a kite-string of some sort. There is a force tending to carry the bird along with the wind which must be overcome somehow, and I still fail to understand how the albatross can sail in the air indefinitely without some muscular effort.

From Mr. Skye's table, in connection with the other, we get this important practical result-that in a flying-machine, properly constructed, the greatest power required will be that necessary to lift it from the ground; and that once off, up to a certain limit, the stiffer the breeze the better.

The efficiency of a propeller of any sort will depend not only upon its area, but also upon its ability to send the air away in parallel streams. If we wish to go forward, the air must be driven aft, and a forced current in any other direction will at best give us back but a fraction of its energy. Ordinary screw-propellers have not proved very effective, for the reason, probably, that revolving at great speed, they send off a large amount of air tangentially.

What, now, should be the mechanical construction of a successful flying-machine? How should it be built? In what way should the power be applied? I have tried to make clear what seem to me the principles involved, but the best method in which to apply them can only be found by patient and intelligent study and experiment. Many men have been and are now working at the problem, and that it will be eventually solved seems certain. A bird's muscles, while strong, are not as strong as steel, and while his power in proportion to his weight is great, we can exceed it; and let us not admit that we can not equal his intelligence in applying it.

One of our illustrations shows the flying-machine invented by Mr. Henson in England in 1842, and deserves mention as being the first

of importance designed to fly without the aid of muscular power. The chief feature was the very great expanse of its sustaining planes, which were larger in proportion to the weight than in many birds. The machine advanced with its front edge a little raised, and the air acting upon the lower surface, when the proper speed had been attained, was expected to lift and sustain it. This speed at the start-off was to be got by running down an inclined plane or hill, and the object of the screw-propeller was simply to keep up the motion. It is unnecessary to say that this machine did not work, and yet Henson evidently had a glimmering of what is required. He introduces the inclined plane and propeller, but does not apply them in a practical way. Such a machine, of course, would be completely at the mercy of the winds; and while he might find a convenient hill to roll down in order to get the required velocity, in coming to earth again there might be trouble.

Landell's flying-machine, invented in 1863, was also provided with an extensive aëro-plane, but differs in having screws acting vertically to sustain the machine in addition to those for driving it forward. Capping all are two parachutes, intended to open and prevent a sudden fall in case of accident. There are four sets of blades on each vertical screw-shaft, on the principle, one would think, that if one set would be a good thing, four sets would be four times as good. They