Captain Bill interrupted. “Just a minute, just a minute. You say those things pretty glibly. Do you know what they mean? What’s a tachometer? Pat here doesn’t know.” Bob looked embarrassed. “Well, they’re all pretty necessary instruments. I’ve been meaning to look them up, that is, Gee, I really ought to know, oughtn’t I?” “You ought,” said the Captain severely. “Do you mind if I interrupt your story for just a minute and give you a few pointers? This is mostly for you and Hal. You’ll never be able to fly unless you understand what the instruments on the dashboard are for. Of course a lot of the old flyers, like Patrick, here, flew just by instinct, and stuck their heads out over the cockpit to see what was happening. A real pilot nowadays, though, can be sealed in his cockpit and never see ahead of him from the time he takes off until he lands, just so long as his instruments are working. He can keep his course over any country, no matter how strange. You’ve got to know your instruments.” “Well, tell us,” said Bob. The Captain sat up. “I guess the first thing that Lindy watched was the tachometer. This is the instrument that shows the number of revolutions per minute, or R. P. M.’s that the engine is making. A flyer must know how many R. P. M.’s his engine must make to maintain a correct flying speed, or he’ll go into a stall, which is bad. I’ll tell you more about stalls later. The altimeter registers the height at which the plane is flying. It isn’t very accurate at low altitudes, but it’s all right higher up. You soon learn by the feel and the lay of the land how high up you are. The exact height doesn’t matter in ordinary flying, just so that you keep a good altitude. Then there’s that most important instrument, the earth inductor compass. This is much more accurate than a magnetic compass, and it keeps the ship on its course. It operates in regard to the electro-magnetic reactions of the earth’s field, and directions are indicated in reference to magnetic north. To steer by this compass, you have to set your desired heading on the controller, and then steer to keep the indicator on zero. If you veer to the left, the indicator will swing to the left, and to keep on your course you must bring your plane back to the right. When he changes his course, the pilot consults his maps and graphs, and makes a change in the indicator of the compass. “Then there is the air speed indicator, which shows the speed of the plane in the air. This is necessary so that the engine is not over-speeded. A pilot never runs his plane at full speed as a general thing, because he’ll wear out his engine. He keeps it at about 80 per cent of its potential speed, which is a good safe margin.