Water-spouts are formed by two winds blowing in opposite directions, and raising or sucking up the water in their vortex. They generally form a double cone; the superior part with its apex downwards, consisting of a dense cloud, while the inferior cone, the apex of which is turned upwards, consists of water, which is thus sometimes raised to a height of several hundred feet.
Water-spouts seldom last longer than half-an-hour. Their course and movements are irregular; straight forwards; in zig-zag lines; alternately rising and falling; stationary; slow; or progressing with the rapidity of thirty miles an hour. The rotatory movement is also variable; its power is often very great, but sometimes water-spouts pass over small vessels without injuring them. They are more frequent near the coast than on the high seas; and are more commonly seen in warm climates. They seem to occur particularly in regions where calms frequently alternate with storms, which is not to be wondered at, since they owe their origin to miniature storms or whirlwinds.
How do the aqueous vapours with which evaporation impregnates the atmosphere, again descend upon the surface of the earth?
Everybody knows that when in summer a bottle filled with cold water is brought into the room, it soon gets covered with thick dew-drops, which presently trickle down its sides, although it was perfectly dry on entering. Whence does this moisture come from? Not from the inside of the bottle as ignorant people might imagine, but from the surrounding atmosphere; in consequence of the capacity of the air to absorb and retain moisture, increasing or diminishing, as its temperature grows warmer or colder.
Thus when the cold bottle is introduced into the room, the warm sheet of air, which is in immediate contact with its surface, immediately cools, and being no longer able to retain all the moisture with which it was impregnated, is obliged to deposit it on the sides of the vessel. This familiar example suffices to explain the formation of dew, rain, hail, snow, hoar-frost, and all other atmospherical precipitations. They all result from the influence of some refrigerating cause upon the air; such as the passage of a warm current into a cooler region; the influx of a cold wind; a cold-radiating chain of high mountains; a forest, and so forth.
The very name of dew is refreshing, and calls forth a host of pleasing ideas, associated as it is with the memory of serene skies and sunny mornings. How beautiful are its diamonds glittering in all the colours of the rainbow, on verdant meads, or on the blushing petals of the rose. How suggestive of all that is lovely, pure, and innocent!
Poetry is of older date than prose, and bards have sung long before philosophers inquired. Thus, although the children of song from Homer and Theocritus to Byron and Wordsworth so frequently mention dew in their immortal strains, it is only in our time that its formation has been fully explained by Dr. Wells, who in a very ingenious and masterly essay on this subject, first proved that it results from the ground radiating or projecting heat into free space, and consequently becoming colder than the neighbouring air. During calm and clear nights, the upper surfaces of grass-blades, for instance, radiate their caloric into the serene sky, from which they receive none in return. The lower parts of the plant, being slow conductors of heat, can only transmit to them a small portion of terrestrial warmth, and their temperature consequently falling below that of the circumambient atmosphere, they condense its aqueous vapours. Clouds on the contrary compensate for the loss of heat the grass sustains from radiation, by reflecting or throwing back again upon the terrestrial surface, the caloric which would else have been dissipated in a clear sky, and this is the reason why dew does not fall, or but slightly falls during clouded nights. It is easy to conceive why none is formed in windy weather, as then the air in contact with the ground is constantly removed ere it has time to cool so far as to compel it to part with its moisture. We can also understand why dew is more abundant in autumn and spring than at any other season; as then very cold nights frequently follow upon warm days; and why it is most copious in the torrid zone, as in those sultry regions the air is more saturated with moisture than anywhere else, and the comparatively cold nights are almost constantly serene and calm. Hoar-frost is nothing but congealed dew, and owes its formation to the same causes.