The lieutenant, when approached, admitted that they were now in for double duty. "Every man in the corps must report and be ready in the morning. There will be two days of observation, and then look out for some business."

"When will we go to Verdun?" asked Ralph.

"We are expecting orders at any moment," was the reply.

During the previous afternoon more than fifty machines had a.s.sembled, and some of those were now leaving, a few going directly east, on observation tours, while others were circling about and testing the engines.

"I am expecting a big Farman machine," said Lieutenant Guyon, as he returned from headquarters an hour afterwards, "that is equipped for carrying bombs, but I am told it is one of the speediest machines in the service. It will easily carry six hundred kilos in bombs (1,200 pounds), and we are ordered to wait here until it arrives."

When the boys were alone, Alfred, with a glint of joy in his eyes, remarked: "I think that will give us an opportunity to do some flying with the lieutenant."

"Do you think so?" replied Ralph, elated at the thought.

"I am sure of that," said a voice behind them.

They quickly turned and saw the lieutenant, who had entered un.o.bserved.

They saw by the smile on his face that the remark was an agreeable one, for he continued:

"Yes, and the machine is now here; the men are at work setting it up; so we might as well go over and help out."

A second invitation was not necessary. Ralph, who was outside in one bound, rushed across the field, but Alfred accompanied the lieutenant to the commandant's office, where they were provided with the receipt for the machine. The lieutenant remained here while Alfred carried the doc.u.ment to the warehouse. Within two hours the machine was in condition to receive the fuel and the supplies usually carried in the machines for emergencies.

When the lieutenant returned he made a careful inspection, and on this occasion the boys followed every movement of his to learn what were the essential requirements in inspecting. Naturally, the most important thing was to know that every part of the frame is not only properly set up and the wires made taut, but that the fastenings, the turnbuckles, are in good condition. That necessitated a minute examination of every one of them.

Then the planes were sighted to ascertain whether they were properly aligned. Sometimes when the planes are not exactly parallel with each other, the end of one, for instance, being set a little higher or lower than the other, it is usually corrected by letting out one or more sets of brace wires and taking up on others. This observation was followed by a careful look at the control planes. These are the sensitive parts of the plane, and may be likened to feelers, for the slightest warping of the horizontal tail planes will frequently cause the machine to fly with a skidding motion, not dangerous, but exceedingly uncomfortable when flying at a high rate of speed.

"The wires leading to the control levers are always a source of worry to me," remarked the lieutenant, and he drew himself up into the rear part of the cha.s.sis. "You will notice that this one rubs along the side of that brace. I could not permit that," he said, as he glanced toward one of the workers.

"Do you think it would weaken the brace?" asked Ralph.

"No, it's not that," replied the lieutenant. "The difficulty is that all these wires are stranded, and as soon as one of the wires wears out by the frictional contact, another will give way, and the control wire is liable to part at the most critical time."

The boys had occasion to remember the lieutenant's warning shortly thereafter. After an examination of the airplane structure an investigation was made of the engine. The mechanic turned it over to be sure that the compression was all right. Self-starters are provided on all these machines, as the French learned at an early day that it would be unwise to depend on cranking.

Then a thorough inspection of the pipes and tubing for the fuel and lubrication was made. The dashboard of an aeroplane is a much more wonderful contrivance than the dashboard of an automobile. The aviator must look out for several things not necessary when traveling on land.

The machine which they were examining had on it the most improved contrivances, which had been found desirable, some of which were entirely new to the boys, one of them being the inclinometer.

"That must be a dandy thing," said Alfred, pointing to it.

"I think there are only a few of the military machines which are supplied with them, but the order has been given to put them on all the new machines," said the lieutenant.

"What are the two rods for?" asked Ralph, as he looked at the inclinometer.

[Ill.u.s.tration: FIG. 6. INSTRUMENT BOARD OF STANDARD AIRPLANE.

1. Control Lever, with thumb switch. 2. Watch. 3. Altimeter, for registering height. 4. Compa.s.s. 5. Pressure Gages for two gasoline tanks 6. Dial to register engine revolutions. 7. Switches for two magnetos. 8.

Air Speed indicator. 9. Clinometer. 10. Oil Pulsator. 11. Gasoline supply system. 12. Engine crank and fly-wheel.]

"The two rods which swing past the vertical bar, on which you see the numbers, indicate the angle at which the machine is going. All that is necessary is to observe the crossing point of the rod over the marked bar, and it will instantly tell what the angle of ascent or descent is.

You see an aviator now has so many things to think of that he must have devices of various kinds which will constantly tell him anything or any condition without asking for it."

"Why, this also tells how fast the engine turns," remarked Alfred.

"Of what use is that?" asked Ralph.

"Well; if you knew that the engine was capable, under full throttle, to make eighteen hundred revolutions a minute, and the dial showed that the shaft was turning only one thousand, you would quickly get the idea that something was wrong," answered the lieutenant.

"Oh, yes; and then you would start to make an investigation at once without waiting for the engine to stop," said Alfred.

"Exactly; it is there as a warning, just as a headache is a warning that something is wrong, or a pain, which is nature's way to indicate that an investigation should be made without delay," answered the lieutenant with a nod.

Besides the foregoing, the dash had on it a watch. Now, a watch may be a convenient thing to have on an automobile, but it is not a necessity. On a scouting and observation plane it is one of the necessary implements.

Alfred laughed, as he looked at the neat little clock face.

"I suppose," he said, "we could get along without that."

"Of course we could," answered the lieutenant. "But why get along without it when we now find it such a necessary element?"

"Necessary how?" asked Ralph, in surprise.

"In determining distances, for one thing," answered the lieutenant.

"A watch to tell the distance?" asked Alfred incredulously.

"Yes, indeed," responded the lieutenant. "If you saw a puff of smoke in the distance, and shortly thereafter heard a boom, the distance could be determined almost instantaneously by roughly calculating 1,200 feet for every second. You see, the clock there has a second hand, which is very plain, for that very purpose."

"But suppose there is a regular battle on, it would be hard to tell about the boom from any particular gun, wouldn't it?" asked Ralph.

"Quite true; in such a case it would be useless for that purpose.

Another use is in signaling," continued the lieutenant. "For instance, in sending information to a battery, arrangement is sometimes made to flash the distance by means of second intervals."

"How is that done?" asked Alfred, now growing intensely interested in the details of the dashboard mechanism.

"It is an easy matter to signal numbers," answered the lieutenant and a favorite plan is to fly over the area where the enemy's battery is located, then flash at that alt.i.tude. The gunner will take the angle, set his piece and fire, the aviator, meanwhile, noting the course and effect of the shot. By means of the watch he can determine the distance, counting either by the time of flight of the missile, or by the elapsed time between the flash of the gun and the landing of the shot.

The altimeter is another very important part of the equipment. It is a device which tells at a glance how high the machine is flying, and Ralph looked at it with a somewhat doubtful expression. The lieutenant saw the questioning lines on his face, and instantly divined the reason.

"You are in doubt whether the altimeter can be relied on? Am I correct?"

he asked.

"Well, I suppose it is all right, but almost every time we have been up and took the readings, we found we were actually hundreds of feet less than the altimeter actually showed, so I am wondering if it is possible really to tell the exact height by that apparatus," remarked Alfred.

"Yes, I understand what you mean," replied the lieutenant. "The other day, when we made the long flight from Bar-le-Duc, the device we had showed 900 meters, for it was an automatically registering barometer, as all of them practically are. When we returned I noted on the report book 872 meters. Did you notice that?"

"Yes, and I wondered at it at the time," said Alfred.

"I knew that the general alt.i.tude of the country at that place, as indicated by the topographical charts, was 28 meters above sea level so I merely subtracted 28 from 900, and that showed how far up above the land we really were," said the lieutenant.