EXPLOSIVE ENGINES AND INFLAMMABLE GASES
I HAVE been so often and so sincerely warned against what is taken for granted to be the patent danger of operating explosive engines under masses of inflammable gases that I may be pardoned for stopping a moment to disclaim undue or thoughtless rashness.
Very naturally, from the first, the question of physical danger to myself called for consideration. I was the interested party, and I tried to view the question from all points. Well, the outcome of these meditations was to make me fear fire very little, while doubting other possibilities against which no one ever dreamed of warning me.
I remember that while working on the first of all my air-ships in that little carpenter shop of the Rue du Colisée I used to wonder how the vibrations of the petroleum motor would affect the system when it got in the air.
In those days we did not have the noiseless 
THE QUESTION OF PHYSICAL DANGER
My tandem motor of two cylinders, working the same connecting-rod and fed by a single carburator, realised 3 horse-power—at that time a considerable force for its weight—and I had no idea how it would act off terra firma. I had seen motors "jump" along the highway. What would mine do in its little basket, that weighed almost nothing, and suspended from a balloon that weighed less than nothing?
You know the principle of these motors? One may say that there is gasoline in a receptacle. Air passing through it comes out mixed with gasoline gas, ready to explode. You give a whirl to a crank, and the thing begins working automatically. The piston goes down, sucking combined gas and air into the cylinder. Then the piston comes back and compresses it. At that moment an electric spark is struck. An explosion follows instantly; and the piston goes down, producing work. Then it goes up, throwing out the product of combustion. Thus with the two cylinders there was one explosion for every turn of the shaft.
Wishing to have my mind clear on the question I took my tricycle, just as it was after I had left the Paris-Amsterdam race, and, accompanied by a capable companion, I steered it to a lonely part of the Bois de Boulogne. There in the forest I chose a great tree with low-hanging limbs. From two of them we suspended the motor tricycle by three ropes.
When we had well established the suspension my companion aided me to climb up and seat myself on the tricycle saddle. I was as in a swing. In a moment I would start the motor and learn something of my future success or failure.
Would the vibration of the explosive engine shake me back and forth, strain at the ropes until it had unequalised their tension, and then break them one by one? Would it jar the interior air balloon's pump and derange the big balloon's valves? Would it continually jerk and pull at the silk hems and the thin rods which were to hold my basket to the balloon? Free from the steadying influence of the solid ground, would the jumping motor jar itself until it broke? And, breaking, might it not explode?
All this and more had been predicted by the professional aeronauts, and I had as yet no proof outside of reasoning that they might not be right on this or that topic.
I started the motor. I felt no particular vibration, and I was certainly not being shaken. I increased the speed, and felt less vibration! There could be no doubt about it—there was less vibration in this light-weight tricycle hanging in the air than I had regularly felt while travelling on the ground. It was my first triumph in the air!
I will say frankly that as I rose in the air on my first trip I had no fear of fire. What I feared was that the balloon might burst by reason of its interior pressure. I still fear it.
Before going up I had minutely tried the valves. I still try them minutely before each of my trips. The danger, of course, was that the valves might not work adequately, in which case the expanding of the gas as the balloon rose would cause the dreaded explosion. Here is the great difference between spherical and dirigible balloons. The spherical balloon is always open. When it is taut with gas it is shaped like an apple; when it has lost part of its gas it takes the shape of a pear; but in each case there is a great hole in the bottom of the spherical balloon where the stem of the apple or the pear would be, and it is through this hole that the gas has opportunity to ease itself in the constant alternations of condensation and dilatation. Having such a free vent, the spherical balloon runs no risk of bursting in the air; but the price paid for this immunity is great loss of gas and, consequently, a fatal shortening of the spherical balloon's stay in the air. Some day a spherical balloonist will close up that hole; indeed, they already talk of doing it.
I was obliged to do it in my air-ship balloon, whose cylindrical form must be preserved at all cost. For me there must be no transformations as from apple to pear. Interior pressure only could guarantee me this. The valves to which I refer have since my first experiments been of all kinds—some very ingeniously interacting, others of extreme simplicity. But their object in each case has always been the same: to hold the gas tight in the balloon up to a certain pressure and then let only enough out to relieve dangerous interior pressure. It is easy to realise, therefore, that should these valves refuse to act adequately the danger of bursting would be there.
This possible danger I acknowledged to myself, but it had nothing to do with fire from the explosive motor. Yet during all my preparations, and up to the moment of calling: "Let go all!" the professional aeronauts, completely overlooking this weak point of the air-ship, continued to warn me against fire, of which I had no fear at all!
"Do we dare strike matches in the basket of a spherical balloon?" they asked.
"Do we even permit ourselves the solace of a cigarette on trips that last for many hours?"
To me the cases did not seem the same. In the first place, why should one not light a match in the basket of a spherical balloon? If it be only because the mind vaguely connects the ideas of gas and flame the danger remains as ideal. If it be because of a real possibility of igniting gas that has escaped from the free hole in the stem of the spherical balloon it would not apply to me. My balloon, hermetically closed, except when excessive pressure should let either air or a very little gas escape through one of the automatic valves, might for a moment leave a little trail of gas behind it as it moved on horizontally or diagonally, but there would be none in front where the motor was. (See Fig. 4.)
In this first air-ship I had placed the gas escape valves even farther from the motor than I place them to-day. The suspension cords being very
long I hung in my basket far below the balloon. Therefore I asked myself:
"How could this motor, so far below the balloon, and so far in front of its escape valves, set fire to the gas enclosed in it when such gas is not inflammable until mixed with air?"
On this first trial, as in most since, I used hydrogen gas. Undoubtedly when mixed with air it is tremendously inflammable—but it must first mix with air. All my little balloon models are kept filled with hydrogen, and, so filled, I have more than once amused myself by burning inside them, not their hydrogen, but its mixture with the oxygen of the atmosphere. All one has to do is to insert in the balloon model a little tube to furnish a jet of the room's atmosphere from an air pump and light it with the electric spark. Similarly, should a pin-prick have made ever so slight a vent in my air-ship balloon, the interior pressure would have sent out into the atmosphere a long thin stream of hydrogen that might have ignited had there been any flame near enough to do it. But there was none.
This was the problem. My motor did undoubtedly send out flames for, say, half-a-yard round about it. They were, however, mere flames; not still-burning products of incomplete combustion like the sparks of a coal - burning steam-engine. This admitted, how was the fact that I had a mass of hydrogen unmixed with air and well secured in a tight envelope so high above the motor to prove dangerous?
Turning the matter over and over in my mind I could see but one dangerous possibility from fire. This was the possibility of the petroleum reservoir itself taking fire by a retour de flamme from the motor. During five years, I may here say in passing, I enjoyed complete immunity from the retour de flamme (sucking back of the flame). Then, in the same week in which Mr Vanderbilt burned himself so severely, 6th July 1903, the same accident overtook me in my little "No. 9" runabout air-ship just as I was crossing the Seine to land on the Ile de Puteaux. I promptly extinguished the flame with my Panama hat . . . without other incident.
For reasons like these I went up on my first air-ship trip without fear of fire, but not without doubt of a possible explosion due to insufficient working of my balloon's escape valves. Should such a "cold" explosion occur, the flame-spitting motor would probably ignite the mass of mixed hydrogen and air that would surround me; but it would have no decisive influence on the result. The "cold" explosion itself would doubtless be sufficient. . . .
Now, after five years of experience, and in spite of the retour de flamme above the Ile de Puteaux, I continue to regard the danger from fire as practically nil; but the possibility of a "cold" explosion remains always with me, and I must continue to purchase immunity from it at the cost of vigilant attention to my gas escape valves. Indeed, the possibility of the thing is greater technically now than in the early days which I describe. My first air-ship was not built for speed — consequently, it needed very little .png)
"No. 9" CATHES FIRE OVER THE ILE DE PUTEAUX