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. [Illustration: FIG. 6. INSTRUMENT BOARD OF STANDARD AIRPLANE. 1. Control Lever, with thumb switch. 2. Watch. 3. Altimeter, for registering height. 4. Compass. 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.