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The transparency of the air is a matter which affects every organization engaged in transportation; the impairment of visibility has led to wrecks without number upon land and sea. Within the past few years wrecks from this cause have become a serious menace to air transportation.

Standards for testing the transparency of the air and for measuring the impairment of visibility are used here and there, but there is no uniformity among them; for the greater part they are local as to usage. A few, such as the visibility of the sun's disk, the variability of certain stars, and the sharpness of a shadow cast by a rod on white paper, are pretty general so far as overhead observations are concerned. Seamen all over the world are pretty apt to judge visibility by the clearness of the horizon; and the principle of camouflage is not so much to conceal a vessel as to blend it with sea and sky so that its outlines are indistinguishable. The locomotive engineer gauges visibility according to the plainness with which he can see semaphores during the day and signal lights by night. The marine pilot must be able to distinguish the colors of code flags and smoke stack markings, as well as colored lights. The air pilot must be able to discern the condition of the atmosphere by the refraction of the light passing through it or reflected from it. For almost all purposes, the problems of visibility must be determined along horizontal lines. The impairment of visibility along vertical lines becomes a danger when an airman cannot see his landing place.

Good visibility is safety; poor visibility is danger.

Several factors are concerned in the change from good to poor