Page:The American Cyclopædia (1879) Volume VI.djvu/69

 DEW 61 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 ther- mometer 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 Avould cause the thermometer to rise and fall as they screened the locality or left it exposed to the clear sky. In consequence of the differ- ent 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 dif- ference 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 follow- ing 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 resem- ble 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, whe- ther they be above or on one side of the be- dewed 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 sub- stances that are poor conductors of heat, and yet cool rapidly by radiation, such as glass, cotton, flax, hair, down, &c., 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, &c., act as do solid screens to diminish the deposi- tion 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, &c. ; the least abundant in the interior of dry continen- tal 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 for- mation of dew. 11. The accurate measure- ments 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 precipi- tation 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, es- pecially when heavy. 14. The electrical con- dition of a body has no influence on its capa- city for receiving dew. 15. Objects a few inches above the surface of the soil collect more dew than those lying on the ground it- self. 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 modifica- tions, 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 ex- posed to unobstructed radiation into space loses its heat entirely, except in so far as this is sup- plied by conduction from the interior of its mass, or by convection through the action of suffi- ciently 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 begin!? 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 hu- midity. Thus a continual supply of moist air is maintained, and this is favored by a very slight general movement of the air ; while, on