These deserts, cut through from north to south by a silent river and from east to west by two noisy railways, seem remarkable for only a few commonplace things, according to the consensus of public opinion. All that one hears or reads about them is that they are very hot, that the sunlight is very glaring, and that there is a sand-storm, a thirst, and death waiting for every traveller who ventures over the first divide.
Sunlight on desert.
Glare and heat.
There is truth enough, to be sure, in the heat and glare part of it, and an exceptional truth in the other part of it. It is intensely hot on the desert at times, but the sun is not responsible for it precisely in the manner alleged. The heat that one feels is not direct sunlight so much as radiation from the receptive sands; and the glare is due not to preternatural brightness in the sunbeam, but to there being no reliefs for the eye in shadows, in dark colors, in heavy foliage. The vegetation of the desert is so slight that practically the whole surface of the sand acts as a reflector; and it is this, rather than the sun’s intensity, that causes the great body of light. The white roads in Southern France, for the surface they cover, are more glaring than any desert sands; and the sunlight upon snow in Minnesota or New England is more dazzling. In certain spots where there are salt or soda beds the combination of heat and light is bewildering enough for anyone; but such places are rare. White is something seldom seen on desert lands, and black is an unknown quantity in my observations. Even lava, which is popularly supposed to be as black as coal, has a reddish hue about it. Everything has some color—even the air. Indeed, we shall not comprehend the desert light without a momentary study of this desert air.
Pure sunlight.
Atmospheric envelope.
The circumambient medium which we call the atmosphere is to the earth only as so much ground-glass globe to a lamp—something that breaks, checks, and diffuses the light. We have never known, never shall know, direct sunlight—that is, sunlight in its purity undisturbed by atmospheric conditions. It is a blue shaft falling perfectly straight, not a diffused white or yellow light; and probably the life of the earth would not endure for an hour if submitted to its unchecked intensity. The white or yellow light, known to us as sunlight, is produced by the ground-glass globe of air, and it follows readily enough that its intensity is absolutely dependent upon the density of the atmosphere—the thickness of the globe. The cause for the thickening of the aërial envelope lies in the particles of dust, soot, smoke, salt, and vapor which are found floating in larger or smaller proportions in all atmospheres.
Vapor particles.
Clear air.
In rainy countries like England and Holland the vapor particles alone are sufficiently numerous to cause at times great obscurity of light, as in the case of fog; and the air is only comparatively clear even when the skies are all blue. The light is almost always whitish, and the horizons often milky white. The air is thick, for you cannot see a mountain fifteen miles away in any sharpness of detail. There is a mistiness about the rock masses and a vagueness about the outline. An opera-glass does not help your vision. The obscurity is not in the eyes but in the atmospheric veil through which you are striving to see. On the contrary, in the high plateau country of Wyoming, where the quantities of dust and vapor in the air are comparatively small, the distances that one can see are enormous. A mountain seventy miles away often appears sharp-cut against the sky, and at sunset the lights and shadows upon its sides look only ten miles distant.