All must have observed during the past year the remarkable appearance of the western sky after the sun had set. Cities were more than once supposed to be burning, reflecting their lurid blaze upon the clouds. The cause of this is still a matter of dispute, but is generally attributed to the presence of star dust, or some minute mineral matter suspended in the higher atmosphere.
It will be remembered that color is not an inherent property of a substance, but depends upon what portion of the light rays it absorbs. Snow is white, as it absorbs none of the prismatic colors, but reflects them all to the eye. Whatever, then, varies the absorbing or reflecting power of an object varies its tints. Thus, objects seen on the horizon are red, because the dense atmosphere has turned aside the violet, indigo, blue, green, yellow and orange, and only the red color reaches the eye.
Observe that the initial letter of the prismatic colors taken in their order make the word “vibgyor.”
Again, were there no atmosphere, there could be no
CLOUDS NOR RAIN.
The moon is destitute of these, or at least that half of it which is always turned toward us. The most powerful telescopes can detect there the presence of neither atmosphere nor cloud.
A most remarkable proof of divine wisdom can be seen in the nice adjustment by which the pressure of the air prevents undue evaporation from the lakes and seas, and at the same time furnishes the medium by which moisture is conveyed to the remotest parts of the earth. The fact that water, in the form of mist or clouds, should float, and not fall in a substance many times lighter than itself, is one of the most wonderful of nature’s phenomena. When shot are dropped into water, we expect that they will sink; yet lead is but eleven times heavier than water, while water is eight hundred times heavier than air.
The following seems to be the most satisfactory explanation of the matter: It is a well known fact that the air has the power to absorb and hold, in an invisible form, a certain amount of moisture. The quantity which it can contain depends upon its temperature. If the air is cooled, it parts with a portion; thus if the grass radiates its heat, dew is deposited upon it; if it is very cold, the frost covers it with sparkling crystals. It is thought that when cooling from any cause takes place in higher altitudes, the atmospheric moisture changes from the invisible to the visible form, and assumes the physical condition of spheroids or vesicles, minute bubbles of water in point of fact, each bubble filled with air. These bubbles, heated by the sun’s rays, would become lighter than the medium in which they float, for the same reason that soap bubbles float while they are warm. In this condition they are drifted along by currents until they reach a colder stratum of air, when they are condensed and fall as rain. If cooled sufficiently, snow would be formed.
Cloud forms are four in number, cirrus, cumulus, nimbus, and stratus, all of which may sometimes be seen at once, in the sky of a summer’s day. At times they float above the loftiest mountains. Gay-Lussac,[15] rising in his balloon to an elevation of 21,600 feet, perceived clouds drifting far above him.
