lished the results of an elaborate and careful series of studies made by him upon the flight of birds, which wrought a revolution in the construction of flying machines. Elastic aëroplanes were advocated by Mr. Brown, elastic aërial screws by Mr. Armour, and elastic aëroplanes, wings, and screws by M. Pénaud. The latter constructed models to fly by three different methods — (1) by means of screws acting vertically upward; (2) by aëroplanes propelled horizontally by screws; and (3) by wings which are flapped in an upward and downward direction. These models were so far successful as to make a considerable degree of progress and offer hints for future guidance. Mr. Henson designed a flying machine in 1841, combining aërial screws with extensive supporting structures. Mr. Wenham, in 1867, thinking to improve upon Mr. Henson, invented what he designated his aëroplanes. Mr. Stringfellow, who was originally associated with Mr. Henson, and constructed a sueeessful flying model in 1847, built a second model in 1868, in which Mr. Wenham's aëroplanes were combined with aërial screws. This model was on view at the exhibition of the Aëronautical Society of Great Britain, held at the Crystal Palace, London, in 1868. It was remarkably compact and light, and obtained the $500 prize of the exhibition for its engine, which was the lightest and most powerful ever constructed. The machine for which it was made was not successful. In 1874 Mr. Moy invented an aërial steamer, consisting of a light, powerful skeleton frame resting on three wheels; a very effective light engine constructed on a new principle, which dispensed with the old-fashioned cumbrous boiler, narrow horizontal aëroplanes, and two very large aërial screws. In its general features Mr. Moy's machine resembled that of Mr. Stringfellow.
Summarizing the methods of flight so far attempted, we have, therefore, (1) dirigible balloons; (2) those forms of apparatus which were intended to sustain or lift their weight by screw propellers revolving on vertical axes; (3) those machines which were intended to sustain their weight on flapping or beating wings; and (4) the aëroplane or aeërocurve contrivances which have been experimented with in recent years, to the practical exclusion of all other classes except the dirigible balloon. To understand the reason for this tendency toward the aëroplane or gliding machine, a brief comparative discussion of the different classes of flying machines is necessary. As already stated, the future utility of the dirigible balloon is still the subject of differences of opinion. Its chief drawbacks are great bulk and extreme frailty, which seem to affect its practical advantages in other respects. Vertical screw machines have much to recommend them, but they present drawbacks which more than counterbalance the advantages. The ability to rise directly into the air from any given spot would be an exceedingly desirable quality, and hence a great many attempts have been made to develop a successful vertical screw machine. Perhaps the greatest stumbling block to success has been that when the surfaces which form the blades of the screws are revolved over one spot they do not give any considerable lifting effect in proportion to the power consumed. It is stated by high authority that where one might from theory expect a lift of possibly 100 pounds per horse-power, the best result the inventor can produce on a practical scale is almost sure to be less than one-seventh of that figure; in fact, the lift with the lightest engines we can build is likely to be but little if any more than the weight of the machine itself. With engines weighing much more than four or five pounds per horse-power it is asserted that practical success with this type of apparatus is not possible. The third class, or the beating wing machines, are subject to the same disadvantages in regard to the enormous power required as those of the vertical screw type. In addition to this, the problem of maintaining a stable equilibrium in windy weather still further seriously complicates matters so much, that it is considered that there is but small hope of practical machines operated on this principle being produced. In conclusion, it is hardly necessary to point out that any combination in a single machine of the salient features of two or more of the classes of machines described tends to complicate rather than to improve the situation.
After thorough investigation and experiment, the objections to the three classes of machines named, which have been briefly outlined above, appear so formidable to the great majority of the foremost workers for mechanical flight to-day that there now appears to them to be but one principle left, and upon this there is based an increasing hope that flight will be accomplished. This principle is the one which underlies the aëroplane or aërocurve; which is that when a thin surface is drawn through the air and is slightly inclined to its path, the equivalent of a pressure is developed on the side which is exposed to the air current — that is, the under side — which is much greater than the driving force which is necessary to produce it. If a surface arched in the line of the motion he substituted for the plane, we have an aërocurve, whose chief advantage is that it has a higher efficiency. Another advantage is that it is not necessary to incline an aërocurve in order to develop a pressure on the hollow side when it is moved through the air. The one advantage which the power machine of the aërocurve type has over the vertical screw is the fact that it can for the reasons just stated convert the relatively small push of the screw propellers into a much larger lifting effect.
Recent experiments with aërocurves may be divided info two classes: (1) Experiments with models and (2) experiments with large devices capable of carrying a man. Perhaps the two most notable experimenters with models have been Sir Hiram Maxim of England and Professor S. P. Langley of the United States. Maxim's experiments have been largely with various forms of aërocurves, with the purpose of determining the most efficient, and the model constructed by him was employed in testing the different surfaces. In a paper written in 1896, Sir Hiram Maxim summarizes some of the principal results of his experiments, as follows:
"My experiments have certainly demonstrated that a steam engine and boiler may be made which will generate a horse-power for every six pounds of weight, and that the whole motor, including the gas generator, the water supply, the condenser, and the pumps may be all made to come inside of 11 pounds to the horse-power. They also show that well-made screw propellers working in the air are fairly efficient, and that they obtain a sufficient grip upon the air to drive
