EARLY FA'S THE DEW.

Otago Witness, Issue 3415, 29 August 1919, Page 60

EARLY FA'S THE DEW.

By H. T. Gibson. Hath the rain a father? Who hath begotten the drops of dew? Book of Job. About two hundred years ago Nardius the scientist let fall a bomb among the established theories concerning the formation of dew. Until his time no one had dared to question the opinion, held by all men of science, that dew fell from the atmosphere. Nardius declared it to be an exhalation from the ground. Since his time many students and observers, doubting his statement, have searched for the correct solution of the problem. To the casual observer it would seem that the matter should be quite a simple-one to handle, as the formation of dew is one of nature's commonest phenomena, but it was not until about the year. 1885 that the truth was discovered. Confusion arose because the ordinary of a healthy plant were for many centuries mistaken for dew. These exudations were the "dew-drops" of the poet and the romantic novelist, the "dew-drops" that were born of blushing flowers to provide a bath for the fairy sprites that hailed the dewy .morn. One hesitates to intrude prosaic facts into the romance of pretty ideas, though it i.e very doubtful whether our poetical friends will ever substitute exudation for dew-drop simply because the former is the correct term! They will continue to sing of the dew-drop, whether the form of moisture to which they refer consists of real dew or of water exuded a cabbage. Why not? "Annie Laurie," perhaps the sweetest song ever sung, loses none of its charm because the poet has it that MJa-xwelltown braes are bonny, Where early fa's the dew. Who is cruel enough to say "Where early ascends the dew '(" But, setting aside poetical inaccuraci.es, let us observe certain facts, and we shall find that Dr Aitken's discovery, made some 34 years ago, settled for all time the question whether the moisture which forms dew really rises or falls. - Select a calm and cloudless evening, and place a thermometer on the surface of the ground, and another thermometer, 'a few inches below the surface. A considerable difference between the two readings will be noticed. The writer has observed as much as 15 degress difference (Fahrenheit), the thermometer lying on the ground invariably reading lower than that below the surface. Therefore, the temperature of the ground a few inches below is higher than that of the air immediately above the surface. The dewpoint, or the temperature at which the formation, of dew begins, is the degree at which the. atmosphere becomes saturated with vapour. While the surface remains above this temperature, any vapours absorbed into the ground during the day must necessarily rise, and the moisture of these vapours will be condensed into dew when it comes into contact with any object whose temperature is lower than the dew-point. Ordinary diffusion causes these rising vapours to become mixed with the lower stratas of air, and hence dispersed. We have but to breathe upon a smooth cold surface to prove that moisture is deposited from the warmer breath. Most of us have noticed that the under surface of road metal is often wet with dew while the upper surface is quite dry. The writer has frequently observed that on dewy mornings the earth lying between railway sleepers is quite moist, while the layer of. earth which has been banked up over the sleepers ie comparatively dry, for the sleeper itself has prevented the rise of moist, vapours. "All doubt upon this matter," says Professor M'Pherson, "is removed by a most successful experiment with a fine balance which weighs to a quarter of a grain. If vapour rises from the ground for any length of time during dewy nights, the soil w r hich gives off vapour must lose weight. To test cut from the lawn a piece of turf six inches square and a quarter of an. inch thick. Place this in a shallow pan and carefully note the weight of both turf and pan with the sensitive balance. To prevent loss by evaporation, the weighing should be done in an open shed. Then place the pan and turf at sunset in the open cut. Five hours afterwards remove and weigh them, and it will be found that the turf has" lost a part of its weight. The vapour which rose from the ground during the formation of the dew accounts for the difference in weight. This weighing test will also succeed on bare ground." A greater quantity of dew is deposited on a surface enclosed beneath a confined space than on one in an open situation. We can observe this on an iron house roof. More dew is deposited on the inside of the roof than on the outside, for the enclosed vapour does not become mixed with any drier air which is always present owing to the agency of diffusion or by the action of the gentlest breath of wind. These agene'es—air movements, however slight, and diffusion—cause moisture to be deposited on the upper and side surface? of objects .as well as on the lower, hence we frequently observe many of these objects to be covered with dew. Shelley, therefore, is justified in giving us the beautiful s ? mile, These eyes which burn through smiles that fade in tear 3 Like stars half-quenched in mists of silver dew. Thick clouds act as a blanket, causing a counter reflection of the sun's heat gained during the day, thus usually keeping the temperature of the air considerably tibove dew-point. Hence the moisture from the earth does not rise, and dew is net formed. Wind, by its intermixing effects', will also prevent the formation of dew. Therefore, we must make our experiments on still nights and when the sky is clear. If dew is formed by the condensation of warm vapour rising from the ground, it would follow that frost, which is frozen dew, should be found deposited on under surfaces more freely

than on upper ones, and to prove that this is what actually happens the observer has only to examine the two surfaces of leaves during a severe frost. To summarise, the sun's heat gained and retained in the earth by day and lost by night, warms the earth vapours which rise into the chilled night air. Surfaces exposing a large proportionate area to the chilling effects must act as condensers and become coated with dew, which in turn, providing the temperature falls sufficiently, freezes and becomes frost. It frequently happens that when the temperature is very low the vapour is condensed into the solid form of frost without first entering the liquid state of dew.