With respect to wind there is an important difference between aircraft and marine craft. Mere strength of wind is not dangerous to an aeroplane, except when starting or landing. An aviator flying above the clouds, with no landmarks in sight by which to gauge his movements, is no more conscious of the actual wind at that level, provided it is steady, than he is of the rotation of the earth on its axis. He feels the wind produced by the motion of his machine through the air--the so-called “relative wind”--but no other. The true wind may be a mere zephyr, or a hurricane blowing 150 miles an hour; the effect is the same on his machine, so far as he is able to observe. On the other hand, a strong wind has a very different effect from a light one upon the course of the aeroplane’s flight with respect to the ground beneath. If a pilot, with no landmarks to guide him, steers by compass for a certain point, and if there is a strong cross-wind of which he is unaware, he will be carried far out of his course; a wind dead ahead or astern will merely affect the speed of his flight, so that he will arrive later or sooner at his destination than he expected. One of the important problems of aeronautics, especially from the commercial point of view, is to prevent aircraft from being driven off their course by the wind when flying with no visible landmarks; i. e., over clouds, fog, the ocean, or an unmapped country. When this problem is solved, pilots will fly above the clouds much more commonly than they do now. The winds at high levels are generally both steadier and stronger than at low. The stronger wind is an advantage or a disadvantage, according to whether it is blowing in the direction of flight or the reverse; but as the winds at different levels generally blow in different directions, a pilot who is independent of landmarks can choose whatever level affords the winds most favorable for his intended journey.