At the present time the only atmospheric condition that seriously hampers flying is fog or low cloud. An aviator flying in a fog or cloud is not only liable to wander far from his course, on account of the unknown leeway of his machine, but he is often in great doubt as to his proximity to the ground. One of the curious effects of such a situation is that the airman loses his sense of the vertical. On land our sense of up and down is determined by the force of gravity, pulling us toward the earth. When riding in a terrestrial vehicle, we are conscious of other pushes and pulls; such, for example, as the jolt that pitches us forward when a train stops suddenly, or the outward thrust that we feel when swinging around a curve. Again, in descending in a lift we seem to lose weight, as if gravity had suddenly grown weaker. On earth all such impressions are corrected by the sight of objects around us; but the aviator enveloped in mist has no such guides, and he often becomes quite confused about the direction of the ground. A turn, which involves banking, increases his confusion. Eventually he may be flying almost upside down without being aware of the fact. Professor Melville Jones, who has been through such experiences, says of the pilot’s confusion: “His first indication that something is wrong is, as a rule, either an increase or a decrease of speed that is not counteracted by the accustomed movements of the controls. A period of wild suspense and utter bewilderment now follows, during which the pilot makes violent efforts to recover control, but without success. The next thing that he realizes, if he realizes anything at all, is that he is either on his back or spinning, and the next thing he knows is that he is out of the clouds with the earth standing up at a ridiculous angle and spinning round like a drunken dinner plate. Happy is he that has plenty of air room under these circumstances.” Spirit-levels and similar instruments are affected by the same disturbances that mislead the pilot in his estimation of the vertical; but fortunately there are certain other devices, due to the exigencies of the war, during which cloud flying was a part of the tactics of the military aviator, which have virtually solved this problem, though their use has not yet become general.