"Of course not, my dear," she said after a moment. Then with a deprecating smile she went away. Mr. Greatorex locked the door.

"Now, Tom," he said, "just _explain_, will you? Begin at the beginning; I want to _know_, you know."

"Well, I've been thinking a lot at odd times about airships and things, and reading up what they've been doing in France and Germany. There's little prospect of making a really serviceable machine out of the old gas balloon; it's far too clumsy; can't make headway against a strong wind; but I didn't see why something shouldn't be done on the lines of the aeroplane. You see, it's easy enough to set the thing going, and even to steer it, when you've got it up in the air; but there are three difficulties: to get it up, to let it down without smashing it to bits, and to keep it from turning somersaults. You can overcome the force of gravity by an arrangement of planes when you keep up a good speed; but if you slacken speed, down you come. And all the aeroplanes that have been invented yet can't rise in the air at any given spot. They either have to be thrown off from some elevated position, or they have to get up a momentum along the ground, running like a motor-car. Then again, the motor machinery has been too heavy; engines haven't been able to exert sufficient horse-power in proportion to their own weight. I've worked it out, and I calculate that no good can be done till you get an engine that'll give you one horse-power to every two and a half pounds of its weight."

"Yes. Well?"

"Well, this model is the result of no end of experiments. It goes, as you see; but besides sailing horizontally, it will lift itself. Look!"

He took up the little machine, released another spring, and the miniature airship went flying to the ceiling, where it remained until the spring ran down.

"All very well," said Mr. Greatorex, unwilling to admit that he was impressed; "but the thing is only a _toy_. There's all the difference in the world between a model and the _real thing_, you know. You could never get a spring strong enough to lift a real machine. I'm not satisfied that you could even get the horizontal motion you're so c.o.c.ksure of, with a machine that would carry _men_."

"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 a.s.sistance 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 alt.i.tude 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 a.s.sistance 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 Dusseldorf, 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 a.n.a.lysis 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 pa.s.sed 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, princ.i.p.ally 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 quant.i.ties 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.

Tom was, however, not yet satisfied with his machine. For one thing he had found it impossible as yet to return to earth with any exact.i.tude at a fixed spot. It was a matter of the nice adjustment of the horizontal with the vertical motion, and after repeated failures Tom comforted himself with the thought that it must be only after long practice that an engine driver could pull his locomotive up to a nicety. Obviously much more practice must be required when the task was infinitely more difficult.

Further, in spite of the jacket around the turbine, the heat generated was still too great to allow of travelling any great distance in safety, and the prospective usefulness of the aeroplane was discounted accordingly.

It was Mr. Greatorex who suggested a possible way out of the difficulty.

"Why not have two engines instead of one?" he said. "If one breaks down-why, there's the other."

"It means more weight," said Tom ruefully, "and therefore less speed and less carrying capacity."

"Well, there's no _hurry_, is there? And as for carrying capacity-_I_ don't intend to tempt the fates, or run the risk of a smash-up like-who was it? Icarus? Thought so."

Tom adopted the suggestion. He replaced his first engine by two somewhat smaller, so that if one became overheated there was the other in reserve. The lifting capacity and the speed of the airship were consequently diminished, but scarcely so much as Tom expected.

So far the experiments had been carried on with perfect secrecy. The enclosure was surrounded by trees, and Tom was always careful not to drive his machine above the level of their tops. But one day, a few months after Schwab's visit, he was careering round, to the mingled admiration and terror of Timothy Ball watching him from the ground, when he was startled by an exclamation that certainly did not spring from the lips of that worthy. Timothy was a good quarter-mile away; the voice appeared to come from a spot almost vertically below the aeroplane.

"By George! Look there, Mops!"

Tom took a hurried peep over. There, below him, in a gap between the trees just beyond the inner fence, stood a tall young fellow in tennis flannels, with light blue cap and tie.

"What is it, Pops?" answered a silvery voice; and a second figure joined the first-a girl in white.

"Why, look! Hanged if it isn't an aeroplane, going like one o'clock, too."

"Oh, how dreadful!" cried the girl, looking up. "I am sure it will fall, and there will be a horrid accident. Oh, do come away, Raymond!"

"Not I! This is hot stuff, Mopsy. By Jove, the fellow can steer the thing. He's making for that shanty over there-and coming down like a lark. I say, Mops, give me a leg-up; I want to have a nearer squint at the machine."

"But, Raymond, it's no business of yours-it's-it's trespa.s.sing!"

"Trespa.s.sing be hanged! We're next-door neighbours. Come, give me a shove up."

He clutched the top of the fence; his sister, still feebly expostulating, gave him a most workmanlike hoist, and in a few seconds he disappeared on the other side. The girl waited a little; then turned and walked away.

Her brother meanwhile was hastening across the field towards the workshop, near which the aeroplane had by this time alighted. Halfway he was met by Timothy Ball, who touched his cap and said-

"Beg pardon, sir, but these are private grounds and you're a trespa.s.ser."

"That's all right. My name's Oliphant; we're neighbours of yours, you know."

"Now, that's a good un. Hold hard, sir"-as the intruder made to walk round him-"my orders is to allow no one on the premises! Your name's Oliphant, you said? Well, you can't be a neighbour, 'cos the only neighbour I knows of is Lord Langside."

"He's my father."

"But-Oliphant--"

"Exactly! Now, come along, my good fellow-I want to see your master."

"He don't want to see you, though. No you don't; keep off, sir; my orders is to allow no one on the premises."

Then began a little game of dodging, Timothy stepping in front of the intruder and stretching his arms like a cattle driver. In half a minute Raymond Oliphant gave it up.

"Really, this is too absurd," he said good-temperedly. "I say, I'll give you my word to stay here while you go and tell your master that I'd like to introduce myself to him."

"Then you'll stay a long time, 'cos master's in town and won't be home till seven o'clock."

"Well then, the driver of that aeroplane, whoever he is. Ah! here he comes, thank goodness!"

Tom was hastening across the field. He wore his usual working suit of blue alpaca; face and hands were much begrimed.

"Your watch-dog here won't let me pa.s.s," said Oliphant with a smile as Tom came up. "I told him I was a neighbour, but he thinks I'm a bad lot.

We've only just come to the place; my father has taken it, you know; he's coming down for shooting as soon as Parliament's up. I saw your aeroplane skylarking round, and couldn't resist the temptation to come over for a nearer look. You don't mind?"