"Of course, it's a satisfaction to know how high one is flying," said Ralph.

"It is not only a satisfaction, but it is an absolute necessity for an aviator to know how high he is flying, if his mission is to convey any information to the batteries in the rear," said the lieutenant.

"And how is that done?" asked Alfred.

"By a system of triangulation," he replied. "In a few days we may have an opportunity to use the system, and I will explain it then so you can quickly grasp the subject."

The air speed indicator was another interesting device, but at that the lieutenant shook his head as though doubting its general value.

"Don't you think it's accurate?" inquired Alfred of the officer.

"Oh, yes; it is as accurate as it can be made. In fact all devices, which are used to indicate the speed of the wind, are nearly perfect.

That is not the difficulty. When it is used on a flying machine several new things enter into the problem, which makes it difficult for the instrument to be truthful."

"What is the main trouble?" asked Alfred.

"Well, suppose we are starting a flight against a wind, which is moving at the rate of, say, ten miles an hour," answered the lieutenant.

"While in flight we note that, going against the wind, the air speed is sixty miles. Now, if we happened to stop suddenly so we were poised in the air, the indicator would show the air speed to be ten miles, so that we were really going only fifty miles. If, now, the machine is turned so that we are going at right angles to the movement of the wind, although we may be going actually through the air at a speed of seventy miles an hour, the indicator does not register the air speed at all, because it is in such a position that the air does not affect it and the only indication we have is the speed of the air relative to the movement of the machine,--or, to put it in a better way, I might say that the instrument only registers, in reality, the speed at which the aeroplane goes through the air, and no more."

"But is that the only way to tell how fast we are going?" asked Ralph.

"The best test is to take the engine revolutions. A certain machine after it has been properly tested in a calm, and also in wind currents of different velocities, will show a record of speed with the engine running at certain revolutions. An experienced aviator can judge pretty well, by experience, what the speed of the wind is, and, knowing the propeller speed, and the direction he is going relative to the wind, he can make a fair estimate of the speed of the machine."

CHAPTER X

A THRILLING EXPERIENCE IN THE AIR

It was with feelings of joy that the boys stepped out of their tent the following morning to learn that the lieutenant had decided on a trial spin in the forenoon, preparatory to the real start, which was to take place at two in the afternoon. The engine was turned over, and the machine held down while the usual hour's running test was going on.

Everything seemed to be in order, the engine was stopped and another inspection made when the lieutenant gave the order to mount the machine.

In this type of military machine the swivel gun is at the extreme forward end, and the pilot directly behind the gunner, and at a slight elevation above the gun. Both gunner and pilot are housed in, and in most cases strapped to the body of the machine to prevent any accidental falling out, as there have been numerous accidents due to men being thrown from their perches.

The boys found ample room in the forward pit, and at a word the machine was released and glided forward. The new machine gave them a sensation different from the former one, as it appeared to ride more easily and steadily. In fact, there is just the same difference in the motion of flying machines as there is in autos, some, the small machines, seeming to glide over the ground and impart every b.u.mp to the occupants, as might be expected of a light machine.

A heavy flying machine, however, seems to move along with the utmost ease, and does not sway from side to side when sailing through cross currents. Naturally, such a machine is a more stable base for machine guns as well as for observation.

Alfred's particular hobby was kodaking. On this occasion, the first time he had an opportunity, he prepared himself with a camera. The lieutenant, learning of Alfred's penchant in this direction, had a regulation government instrument sent over, and attached to the machine, to Alfred's surprise.

Before starting out, the lieutenant said: "I want to give you some points now, in using the camera. Photographs, in order to have any military value, must be identified,--that is, they must be made at such alt.i.tudes, and with reference to certain marked characteristics on the earth, that we may be able to place them on the topographical maps of the particular section, and thus verify them day after day."

"What do you mean by verifying them day after day?" asked Alfred.

"I mean by that to see what changes have taken place in the same view between the taking of the two pictures," replied the officer.

"Well, what possible changes could take place in a day or two?" asked Ralph in astonishment.

"A great many, and very important ones, too," answered the lieutenant.

"The camera sees what the eye does not notice. If a picture is taken of a certain section today, and the following day another picture is snapped, a comparison of the two will show whether or not there has been a movement of troops, or, whether or not a certain undistinguishable object was something movable, or not."

"Is that the way the map of the enemy's country is made up?" asked Alfred.

"Yes, and every detail is noted, so that when you commence the work be careful to notice something which will distinguish the exact place where you are located; also put down the direction of the flight over that same area, and the alt.i.tude at which we are flying."

As the machine glided over the treetops and the village on the northern border of the Meuse, the sun shone out with intense brilliancy. They were going directly east, and both boys turned toward the lieutenant, their eyes seeming to say: "So you are going to take a peep at the front, by way of experiment?" which was exactly what the lieutenant purposed doing.

The altimeter now registered 1800 meters, considerably over a mile high.

From that point they could see to Verdun, and thirty miles east of Verdun, could be noted the blurred outlines and surroundings of Metz, that greatest of all fortified German defences. On they went for nearly three-quarters of an hour until a stream of some magnitude appeared to the east, which they soon crossed.

They afterwards learned that this was the Mosel, which flows through Metz. The latter city now loomed up so it could be plainly seen, and to their right was another town, Nancy, a fortified French place. They were about midway between the two places.

Suddenly an aeroplane appeared to the right, and the lieutenant turned the machine in that direction. The boys were relieved to see the great circle on the control planes of the machine in the distance, which showed them it was a French reconnoitering airplane. In the east several other machines were noticed hovering about, and the lieutenant at first headed directly for them, but he soon turned to the north.

Below were seen numerous flashes of guns, and they knew from this that the fighting lines must be along the river. The most startling thing which occurred was a tremendous explosion that seemed to be right in the machine itself, and they looked around startled at the suddenness of it, for everything seemed peaceful enough to them at that alt.i.tude.

The lieutenant turned the machine, however, as he apprehended more trouble, now that the gunners below had so luckily secured a fair range.

The first shot was followed in another moment by another, and the lieutenant now began to dart the machine back and forth, and turn it in short, sharp circles.

Another explosion took place, this time remarkably close, and pieces of the flying machine came down, and brushed past the boys. The lieutenant shook his head, as the boys glanced back. The machine was now circling around, a peculiar manoeuver, the boys thought, as they looked at each other for a possible explanation.

Ralph looked back, and noticed that the lieutenant, with an alarmed look on his face, was constantly glancing back to the tail planes. The steering lever seemed to be jammed. Ralph had an idea that something was wrong, and crawled over the body until he was alongside of the lieutenant. The latter pointed to the control lever, and Ralph understood.

Without waiting for instructions from the lieutenant, he cautiously made his way along the cha.s.sis, following up the wires which were indicated by the lieutenant. Within four feet of the tail plane Ralph halted.

Then, reaching down, he succeeded in grasping a dangling wire, and held up the end for the lieutenant's inspection. After several attempts he found the other end of the wire, which had been severed by a shot from a shrapnel sh.e.l.l.

The ship was still circling around, so without waiting to repair the break Ralph turned the rudder, and, glancing at the lieutenant, saw the latter smile, and then by motions direct him to give the control plane a still further push in the right direction. It was impossible for Ralph to hear anything the lieutenant said, but as soon as the airplane was fairly headed for the west, the power was shut off, and the machine began to descend.

"Good boy!" shouted the lieutenant. "Can you fasten them together?" he asked.

"I am afraid not," replied Ralph. "A piece seems to have been shot away.

But I can take care of it if you will only direct me to pull it to the right or to the left."

"All right, then. Do you see that great patch of green, directly beyond the castle?" asked the lieutenant.

"I can see it plainly," replied Ralph. "Shall I steer for it?" he asked.

"Oh, no. We must come up from the south. Note the direction of the wind from the smoke. Make a long sweep. We are now at an alt.i.tude of 1200 meters. We can volplane twelve kilometers from this height," remarked the lieutenant.

The turn was made to the south in a long sweep, Ralph continually glancing forward to note the direction indicated by the lieutenant.

"Now straight ahead, and hold steady."

They were now rapidly approaching the earth; the machine gliding over the tops of the trees at the border of the forest that ran along the river bank. To their right, across the river, was a large town, Pont-a-Mousson, and the castle ahead of them was a large school, from which even then hundreds of children were emerging, and excitedly rushing toward the approaching machine.

Down went the machine at a sharper angle; then, when close to the earth, the rear control was sharply turned, and the nose of the ship raised up, but for an instant only, when the machine settled down, and, after running along a hundred feet, stopped.