Dew, the humidity of the air, deposited on cool surfaces with which it comes in contact. It is commonly formed at night upon the leaves of grass and trees and other objects, especially when the sky is clear so as to permit sufficient radiation of heat from them to cool their surfaces, and consequently the layer of air next them, below the point of saturation, or dew point. The moisture which collects upon the surface of a cold body, as a pitcher of ice water, standing in a warm room, and that which collects on a window pane when it is breathed upon, are strictly examples of the deposition of dew, and also the frosted figures which form on windows and stone flaggings of courtyards and walks. In the latter case the solid surfaces have a temperature below the freezing point of water, and therefore the particles of moisture assume a crystalline form during the act of passing from their invisible vapory condition. The figures thus formed owe their variety to the varying degrees of temperature and moisture, and rapidity of deposition, and also to the nature of the surface of the body and its thermal inequalities, which necessarily exist in curves. The different causes of the formation of dew were never clearly understood until the early part of the present century.

The ancients connected its appearance with the intervention of supernatural powers. As it mysteriously appeared when the air was clear and apparently dry, and gathered upon the herbage in sparkling beads, while it avoided the barren and rocky surfaces, they might well look upon it as a special blessing, possessed of wonderful virtues. Hence it came to be prescribed for restoring the charms of youth, and to be used by the alchemists as a solvent of subtle and mysterious powers. The ancients generally entertained the idea that the moon and stars not only poured down cold upon the earth, but also, in some mysterious way, distilled dew. Aristotle was the first to approach a rational explanation, although, from a want of knowledge only obtained by the use of modern philosophical instruments, his explanations contained errors. He believed aqueous vapor to be a mixture Of water and heat; and as it rarely appeared on mountain heights, he supposed it was abandoned by the heat, and left to precipitate itself upon the earth.

He rejected the idea of lunar or astral influence, and maintained that the sun was the prime cause, "since his heat raises the vapor from which the dew is formed as soon as that heat is no longer present to sustain the vapor." In the middle ages philosophers revived the notion that the moon and stars were the cause of dew. Battista Porta showed the erroneousness of these views by instancing several facts, but he discarded the correct part of Aristotle's theory, that dew was condensed vapor separated from the atmosphere, and advanced the idea that it was condensed air itself. Musschenbroek observed that it was deposited more readily on some substances than on others, and therefore correctly inferred that the object itself had an important influence in its formation. This led to a reconsideration of Aristotle's theory, and its adoption, with the modification that instead of its being discharged from the mass of the air it was only removed from the stratum in contact with the object upon which it was deposited. The experiment of placing a cold body in a warm moist atmosphere proved the correctness of this theory.

But it was not till a series of experiments were made by Dr. William Charles Wells (a London physician, though a native of Charleston, S. C), and published in August, 1814, that a comprehensive theory of the formation of dew was publicly promulgated. His experiments were made in a garden in Surrey, near Blackfriars bridge, and will for ever remain as admirable examples of ingenious philosophical investigation. He exposed little pieces of dry wool of equal weights and sizes during the night, their increased weight in the morning showing the amount of dew which had been deposited. The quantity thus collected was found to be greater on clear than on cloudy nights, but there was much difference in the amount deposited on different clear nights. He discovered that the quantity was less, not only when the air was drier, but also when the wool was covered by any kind of a screen, whether this was a board, a piece of cloth, a tree, or a cloud. Supporting a board a few inches above the ground and placing one piece of wool under and another upon it, each weighing 10 grains, he found that the upper piece gained 14 grains, while the under one gained only 4. He discovered, moreover, that when he used thermometers, the greatest deposition of dew always accompanied the lowest temperature; thus, he often found the temperature 9° or 10° lower above the board than under it, and on one occasion the thermometer fell 14° lower when freely exposed upon the grass than when suspended four feet above it.

A piece of cotton in the vicinity of the latter place gained only 11 grains, while a piece of equal weight and size gained 20 grains lying upon the grass. The passing of clouds would cause the thermometer to rise and fall as they screened the locality or left it exposed to the clear sky. In consequence of the different capacities which bodies have of radiating heat, they attain different temperatures, and upon those which radiate and therefore cool the most rapidly, the greatest quantity of dew is deposited. Dr. "Wells found that grass and wood were covered with it, while plates of metal, stones, and gravel walks were free from it. A glass thermometer placed upon a metal plate and exposed to the clear sky was, after a time, moist with dew, while the plate was dry. To ascertain whether this was caused by difference in position or external circumstances, he placed another thermometer having a gilt bulb beside the glass one, when the latter showed a temperature 9° lower than the other. This was because glass is a better radiator than metal, and therefore cools more rapidly. - The following synopsis of the attendant phenomena of dew is compiled from all the recorded observations. 1. The dew falls most abundantly during calm nights. 2. The drops deposited by fogs resemble dew, but differ in that fog wets all bodies indifferently, while dew attaches itself to some in preference to others. 3. Fogs may exist during winds; dew generally disappears if the wind rises. 4. Dew is deposited in preference on surfaces not protected by shelter from exposure to the clear sky. 5. Other things being equal, the quantity of dew deposited in a given time diminishes in proportion as the exposure to the sky is cut off by screens, whether they be above or on one side of the bedewed body. 6. The nature of a body, and especially the smoothness or roughness of its surface, affects the quantity of dew deposited on it.

Thus, leaves of plants receive more than the earth, sand more than compact soil, glass more than metals; and in general substances that are poor conductors of heat, and yet cool rapidly by radiation, such as glass, cotton, flax, hair, down, etc, are most heavily bedewed. 7. Dew is deposited during the entire night, beginning by or before sunset, and continuing until after sunrise; it forms most rapidly about sunrise. 8. A slight movement of the air is very favorable to the production of dew; but moderate or strong winds are not so, though they do not cause it to disappear entirely. 9. Fogs, haze, clouds, smoke, etc, act as do solid screens to diminish the deposition of dew; very low clouds accompanied by strong winds altogether prevent the dew. 10. The most abundant dews are observed on the shores of the oceans, lakes, rivers, etc.; the least abundant in the interior of dry continental plains, and on islands and ships in mid ocean. Ships approaching a coast soon begin to receive a deposit of dew. The clear cool nights of the western coasts of America and Palestine are peculiarly favorable to the formation of dew. 11. The accurate measurements of Wells show that as much as 20 grains of dew may be deposited in a single night on a surface two inches in diameter.

Dr. Dalton estimates the entire amount of annual precipitation in England to equal five inches of rain. 12. The dew is heaviest in the first clear still night after long continued rain, and in general increases with the dampness of the air. 13. A very great difference of temperature between the air next to the ground and that a few feet above accompanies the formation of dew, especially when heavy. 14. The electrical condition of a body has no influence on its capacity for receiving dew. 15. Objects a few inches above the surface of the soil collect more dew than those lying on the ground itself. From all the preceding observations, Wells arrived at an understanding of the origin and the laws regulating the formation of dew; his conclusions are now, with slight modifications, very generally accepted. According to this view, the radiation of heat from the earth's surface into space, which is counterbalanced during the day by the reception of heat from the sun, takes place with most freedom through a clear, dry air, and is counteracted by screens, clouds, and fogs.

The surface of a body exposed to unobstructed radiation into space loses its heat entirely, except in so far as this is supplied by conduction from the interior of its mass, or by convection through the action of sufficiently rapid currents of air. Thus it happens that the air in contact with the exposed surface of a bad conductor, but a good radiator, is cooled to the temperature of its dew point, and begins to deposit its moisture on the cold surface. If the air be absolutely motionless, only a slight deposit will be formed, and that very slowly; but by reason of the increasing density of the increasingly colder air the latter sinks, and is replaced by warmer air having the same humidity. Thus a continual supply of moist air is maintained, and this is favored by a very slight general movement of the air; while, on the other hand, a strong wind conveys too much heat to allow of the cooling of the bedewed body. The hygrometric tables of Glai-sher, Regnault, and others, allow of an accurate prediction as to the quantity of dew that may be deposited, and even enable one to determine whether frost or dew will prevail.

It will thus be seen that the true origin and nature of dew was quite misunderstood until early in the 19th century. - The Dew Point is the temperature at which the vapor contained in the atmosphere condenses into water. It depends upon the amount contained within a given space; the greater this is, the higher will be the degree of heat necessary to retain it in a vaporous condition. When the air is saturated with vapor it is of course at its dew point, and any reduction of its temperature will cause the separation of a portion of moisture. As this process continues the dew point becomes gradually lowered, and although the proportion of moisture held in solution by the air diminishes, the degree of humidity remains the same; for this term is used not to denote the actual proportion of vapor, but the power of sensibly manifesting itself upon any given reduction of temperature. It is often called relative humidity, and is expressed in parts of 100, or the amount which would saturate the air at the particular temperature when the observation is taken.

Thus, at Philadelphia the average humidity of the air is 73, meaning 73 per cent. of the quantity necessary for saturation; at St. Helena it is 88; at Madrid, 62; in parts of India it is sometimes as low as 10; and on the Andes it is often still lower. Air which is heated much above its dew point is regarded as dry, although it may contain a greater proportion of vapor than that which is called humid. In California the dew point is sometimes 78° below the temperature of the air, and among the Andes the difference is often greater. It has been found as much as 45° at Philadelphia, but the usual range is from 10° to 25°. The following table shows the mean relative humidity of the atmosphere in New York from several observations made each day at the Cooper institute, for the Smithsonian institution, by Prof. Morris:

The difference between the dew point and the temperature of the air is called the complement of the dew point. From numerous observations which have been made with Daniell's and Bache's hygrometers, and with the wet-bulb thermometer, a method has been deduced for determining the dew point with sufficient accuracy, by observations made with the latter instrument. The ratio of the complement varies with the temperature of the air: thus, when it is 53° the difference between the dry and wet bulb is one half the complement; at 32° it is one third, and at 26° only one sixth. Tables have been constructed for readily determining the dew point, which may be found in the various works on meteorology. Thus, in the above table it is stated that the relative humidity of the air at New York, June 1, 1873, was 53.13, while the temperature was 76° F. By referring to the table of relative humidity in the article Hygrometey, it will be seen that the complement of the dew point was about 18°; that is, the dew point was about 18° lower than the temperature of the air, or about 58°. (See Evaporation, Heat, and Hygeometry.)