But sometimes, when the air is thick and misty, or there are low-lying clouds and the earth is shrouded in a fog, the pilot flies in an empty void. Nothing is to be seen above, below, or on either hand; he must rely entirely upon his instruments. The altitude meter tells him whether he is rising or descending; his compass tells him he is flying accurately to his goal; while his own ear, and the tell-tale dial of the engine indicator, informs him that his motor is doing its work. So he sits within the hull of his craft, perhaps 5000 feet above the earth--unseen by those below, and himself seeing nothing of the land over which he flies. Should the wind be steady, he has little to do save keep an eye upon his instruments, and listen to the beat of his engine. But if there are gusts and eddies his hand-wheel is busy; each lurch must be corrected by a movement of the ailerons, each dive checked by a pull upon the elevator. The aeroplane compass, nowadays, has been made accurate and reliable; but pioneers found it a dangerous instrument. Vibrations from the engine, and the nearness to the compass of such large masses of metal as the motor and its fittings, made the needle deviate and give false readings. Once, when upon a cross-Channel flight and out of sight of land, a pilot glanced down at his compass and saw that the needle, instead of pointing northward, was spinning slowly round and round; and so it continued until he had by guesswork groped his way to land. But experience showed how a compass may be mounted upon rubber shock-absorbers to guard it against vibration; how fittings that are near it, such as the steering-column and rudder-bar, can be made of brass, or some other non-magnetic metal; while, as to the attraction of the motor, the compass is corrected against this by introducing scraps of metal into its case, so that the needle can be drawn back to an accurate reading from any deviation it may show. But, even with an accurate compass, the cross-country airman may have anxiety in his mind. He may be flying in a side wind; and this, should it be strong and steady, will have an insidious effect upon his path, tending always to drift him sideways even while he keeps his bow upon a compass course. Fig. 66 should make this clear. The airman seeks to fly from A to B, a distance say of 60 miles; so he sets his course due east and passes steadily across country at the rate of 60 miles an hour, the bow of his machine always upon its compass line. But while he is flying, a southerly wind blowing at the rate of 20 miles an hour is pushing him sideways; and, unless he corrects this leeway, he will find himself after an hour’s flying, not at the point B, towards which he imagined he had been making, but at C, 20 miles north of the spot at which he had wished to alight. The wind, blowing at 20 miles an hour across his course, has pushed him these 20 miles to the north, although he has steered accurately eastward.