415. What benefits result from the radiation of heat, &c.?
But for the radiation of heat, we should be subjected to the most unequal temperatures. The setting of the sun would be like the going out of a mighty fire. The earth would become suddenly cold, and its inhabitants would have to bury themselves in warm covering, to wait the return of day. By the radiation of heat, an equilibrium of temperature is provided for, without which we should require a new order of existence.
The amount of heat which our earth receives from the sun, and the economy of that heat by the laws of radiation, reflection, absorption, and convection, are exactly proportionate to the necessities of our planet, and the living things that inhabit it. It is held by philosophers that any change in the orbit of our earth, which would either increase or decrease the amount of heat falling upon it, would, of necessity, be followed by the annihilation of all the existing races. The planets Mercury and Venus, which are distant respectively 37 millions of miles, and 63 millions of miles, from the great source of solar heat, possess a temperature which would melt our solid rocks; while Uranus (1,800 millions of miles), and Neptune (whose distance from the sun has not been determined), must receive so small an amount of heat, that water, such as ours, would become as solid as the hardest rock, and our atmosphere would be resolved into a liquid! Yet, poised in the mysterious balance of opposing forces, our orb flies unerringly on its course, at the rate of 63,000 miles an hour; preserving, in its wonderful flight, that precise relation to the sun, which takes from his life-inspiring rays the exact degree of heat, which, being shared by every atom of matter, and every form of organic existence, is just the amount needed to constitute the heat-life of the world!
CHAPTER XX.
Rain is the vapour of the clouds which, being condensed by a fall of temperature, forms drops of water that descend to the earth.
It is the return to the earth in the form of water, of the moisture absorbed by the air in the form of vapour.
417. Does rain ever occur without clouds?
It sometimes, but rarely happens, that a sudden transition from warmth to cold will precipitate the moisture of the air, without the formation of visible clouds.