“No spring would do it, it is true; but I’ve worked out an application of the principle. It’s well known that a propeller rotating at sufficient speed can be made to lift a weight into the air. Of course we couldn’t set a real airship rotating bodily; no aeronaut’s head would stand it if we could. But, as you saw, my model doesn’t rotate itself. I’ve only made use of the principle—pretty successfully, don’t you think?”
“Just explain to me thoroughly, will you? I want a little more light on the subject.”
Tom took his model, and patiently expounded the mechanical principles on which he had worked. The upshot of this and other conversations was that Mr. Greatorex became first interested, then enthusiastic, and finally determined. He had a workshop erected on a large piece of waste ground nearly a mile and a half square on his estate, and gave Tom carte blanche to get what assistance and spend what money he pleased. Resolving to keep the matter a close secret until the experiments were concluded, he fenced in the enclosure, and gave strict orders that no one was to be admitted to it without Tom’s consent. Tom himself devised a simple means of defending the workshop from prying visitors. Whenever he left it, he set going a strong electric current through the door handle, which was a more effective protection than locks and bolts.
Tom found, as soon as he came to enlarge his model into a practical working size, that none of the motors then on the market was sufficiently powerful in proportion to its weight to give him the necessary lifting force. The electric motor was out of the question, and an adaptation of the latest petrol engine as applied to motor-cars and launches seemed to offer the most likely solution. Even here, however, the march of invention had not gone far enough. The latest petrol motor, it is true, enabled Tom to keep the machine at a constant altitude when once it was in flight; but it failed to raise it from a position of rest. Some other method must be found, and he set his wits to work to discover it.
The first condition of success was, he felt, the discovery of an explosive mixture far more powerful than that offered by petrol, and yet capable of being harnessed and controlled. He had the run of Mr. Greatorex’s chemical laboratories, and the benefit of the practical advice and assistance of the heads of the experimental staff. Scores of preparations were tried, and, for one reason or another, rejected. Where sufficient power was obtained, it was almost invariably found that the mixture was not stable or uniform in its effects. Several explosive mixtures were discovered quite powerful enough for the purpose; but, as Mr. Greatorex’s chief chemist pointed out, all of them were likely to blow the airship to smithereens in the event of any accident to the machinery.
It was some months after the beginning of the experiments when one of the junior chemists came to Tom with the announcement that he had discovered what he thought might be the very substance required. A German firm, Schlagintwert & Co. of Düsseldorf, had placed on the market a few months earlier a powder which, used as a solution, was highly valuable in preparing photographic plates. The exact ingredients of the powder were unknown, although by analysis it had been found to consist of nitrate compounds; but the buyer was warned by a label that it should not be exposed to great heat owing to the danger of explosion. It had occurred to the chemist to mix a little of this powder with petrol. The result was a paste which dried hard, but gave off almost infinitesimal particles of a highly explosive nature, when floating in an air chamber, though the paste itself was not explosive either under heat or shock.
Tom was delighted with the discovery, and at once proceeded to construct an engine suitable to the peculiar properties of the composition. In building the motor he adapted the principle of the turbine to airship navigation. A powerful fan drove the current of air through a number of perforated aluminium plates covered with the paste. The resultant mixture of air and explosive particles passed into the explosion chamber, the intake being controlled by automatic valves connected with the turbine. The explosion of the mixture was brought about by a sparking plug connected with a small electric battery, the sparking being controlled by a cam on the shaft. At each explosion, the gas generated was forced at an enormous pressure through the turbine to the right of the explosion chamber, thus driving the propeller fixed on the shaft.
Tom made his fan serve a double purpose, not only to drive air through the aluminium plates, but to send a current round a jacket on the outside of the turbine and thus keep the latter cool. This was a highly necessary arrangement owing to the enormous heat generated. From the first, indeed, the difficulty of cooling the turbine was the most serious with which he had to grapple. It required months of experiment before the engine could be worked for more than two or three minutes at a time. Gradually, however, by increasing the power of the fan, and constructing the turbine casing and blades of an alloy specially adapted to resist the effects of intense heat, this difficulty was to a great extent overcome.
The airship when completed was not unlike a huge bird with wings outstretched. The body of the bird consisted of the car and engine. The wings were planes of lath strengthened with aluminium, and capable of being inclined at any desired angle by the simple movement of a switch in the car. A large rectangular plane projecting from the rear of the car acted as a rudder, principally for lateral movement, motion upwards and downwards being provided for either by the inclination of the larger planes or by the special screws actuated by the engine. The latter drove two sets of propellers: one fore and aft, giving a horizontal movement, the other below and above the car, giving a vertical movement. Either set of propellers could be thrown out of gear when desired. Tom would have been glad to dispense with the vertical propellers if he could have done so, but he found that the whole force of his engine was necessary to raise the airship from a position of rest. He had not sufficient motive power to enable him to use such an adjustment of oblique propellers as would have ensured simultaneous horizontal and vertical movement.
Mr. Greatorex at once promoted the fortunate young chemist who had discovered the virtues of the Schlagintwert powder, and swore him to secrecy. The parts of Tom’s machine were made to his order by various firms, the work being distributed so that no one firm should be in possession of the complete apparatus; and a few weeks before Herr Schwab’s visit, an aeroplane capable of sustaining the weight of several men was finished, and in it Tom made daily trips about the field. He tested it so frequently that he used considerable quantities of the powder, and it was not surprising that the curiosity of Schlagintwerts was aroused by the large orders that came from one small place for an article that cost a good deal more than its weight in gold.