Under ordinary circumstances the diffusion of smoke is so rapid that it is rarely visible at a distance of more than 40 or 50 miles from the source of pollution. At this distance a dirty-appearing horizon is about the extent of the impairment of seeing. From Chicago to South Bethlehem the region is one of almost continuous manufacture; nevertheless, the combined smoke pollution of the wide region is rarely discernible at the Atlantic Coast.
Refraction of Light.—Rays of light passing through bodies of air differing in density are bent from their original direction. The outlines of objects therefore reach the observer more or less distorted. The blurring of outlines one notices along a railway track is an example. At a distance of half a mile an approaching locomotive appears as a dark mass without outline. The imperfect mixture of warm air and cold air causes the scattering of light. A boss of rock projecting from the coast, or surrounded by greensward, produces a similar effect noticeable to the air pilot.
Refraction of this sort is a menace to safety whenever it conceals the outlines of objects which should be recognizable beyond stopping distance. A locomotive engineer who loses time in order to make certain of semaphore signals, and is censured therefor, is in about as bad a position as one who is disciplined for running past them for the same reason. Usually the judgment that comes with experience enables the engineer to observe the necessary precautions.
The mirage of arid regions, especially of the desert, is disconcerting at times. It hides landmarks which are necessary to the safety of the traveler; along the railways of arid regions it may disconcert trainmen. When it is below the eye of the observer it has the appearance of a distant body of water which reflects the sky.[1] It is observable only when the eye is not more than 4 or 5 degrees above the level of the apparent surface. The angle is so critical that a change of level of 2 feet on the
- ↑ Trained observers in arid regions are of the opinion that the ordinary desert mirage is due to the reflection of light from the plane of contact of two layers of air resting one upon the other. The experience of the author, covering many years in the desert region of western North America, favors this explanation. In his “Light,” Professor Hastings explains it as a case of refraction, and this view is held also by Humphreys.
