From electric changes, from currents of air, from sudden or gradual depressions of atmospheric temperature--be the cause what it may--the condensation of invisible vapour in the atmosphere is perpetually taking place. But the extent and degree of condensation is very variable. Sometimes, or rather very commonly, the quantity of water separated is small, and condensed into such minute particles that they float in the air, higher or lower, forming wreaths of mist, or clouds of various patterns, density, and extent; sometimes they appear at a great altitude, like silvery waves, or snowy fleeces; sometimes at a lower altitude they take the shape of dense irregular masses, presenting to fancy's eye the outline of monstrous animals, of mountains, and castellated rocks; and sometimes the whole horizon is beclouded and dark; we then know that the condensation is increasing. What is the result? A fall of rain, snow, or hail, in which latter case the water is not only condensed into its ordinary form, but into pellets of ice. The water has passed through a stratum of air at the freezing point. Sudden hail-storms in summer may, we think, be classed among phenomena dependent on electric changes and operations in certain atmospheric strata. We have here, then, two processes--one of evaporation, another of condensation; and these, by an express and admirable provision, have a constant tendency to limit each other's operations. Evaporation is increased by heat, but it produces cold in proportion to the rapidity with which it is carried on. Condensation is produced by cold, and at the same time heat is liberated. How happens it that clouds and rain are formed in the higher regions of the air, and not closely round about us? In consequence of certain laws, the air incumbent on the earth's surface, especially in the hotter latitudes, is always under the point of saturation, the air being really dryer than it is at a great elevation. These laws depend on the difference in the tension or elastic force of vapour at different temperatures; this elasticity increases rapidly from the temperature of 32° to 212°, the increase being in a geometrical progression, while the increase of the temperature of the air from a high altitude to the earth is in arithmetical progression. Now the air of our mixed atmosphere is the ingredient which controls the whole mixture. The result is, that the quantity of vapour present in a mixed atmosphere will, at any successive diminution of height above the surface of the earth, become successively less and less than that which would be required to saturate the air. The consequences of this result are most important, and are connected with our general well-being, and the due exercise of some at least of our senses.