FROST. In the article Dew we noticed frost incidentally. Frost is crystallized vapor, and can only fall on clear, still nights, when the atmos phere is in repose, and the radiation strong. Thus, the atmosphere as it is robbed of its heat, settles by gravitation in strata, the heaviest be e'iming the lowest. Dew is formed, and, if the temperature sinks below forty degrees, frost occurs. The new researches into the phenomena of heat, which have overturned the old hypothe sis of caloric and substituted the theory of vibra tions have brought to light the extraordinary fact that vapor of water is opaque to the rays of heat of low intensity, such as that which pro ceeds from the soil and from plants by night; in other words, that the heat of the earth can not be radiated or projected towards the sky if there exist in the air above the spot observed a large proportion of aqueous vapor. Through pure air, free from moisture, the heat may pass off as readily as if no air existedabove the cooling re gion. It is believed that air saturated with mois ture at the ordinary temperature absorbs more than five hundredths of the heat radiated from a metallic vessel, filled with boiling water, and Prof. Tyndall calculates that of the heat radiated from the earth's surface warmed by the sun's rays, one-tenth is intercepted by the aqueous vapor within ten feet of its surface. Hence the powerful influence of moist air upon the climate of the globe. Like a covering of glass, it allows the sun's rays to reach the earth, but prevents, to a great extent, the loss by radiation of the heat thus communicated. In accordance with this theory, it should be shown that the withdrawal of the sun from any region over which the atmos phere is dry, would be followed by quick refrig eration. It is said that the winters of Thibct are rendered almost unendurable from an uninter rupted outward radiation, unimpeded by aqueous vapor, and that everywhere the absence of the sun favors powerful radiation when the air is dry. The removal for a single summer night of the aqueous vapor from the atmosphere that covers England would, says Prof. Tyndall, be attended by the destruction of every plant which a freezing-temperature would kill. In the Sahara, where the soil is fire and the wind is flame, the refrigeration at night is painful to bear, so that ice is sometimes formed there. In short, says the Professor, it may be safely pre dicted that wherever the air is dry the daily ther mometric range, or the difference between the extremes of heat and cold, will be very great. All great discoveries have been partially antici pated by keen observers, who could not wholly explain certain anomalous appearances, but whose shrewdness led them beyond the borders of the unknown. These results of Prof, Tyndall were thus foretold by R. Russell, Esq., of Scot land, who visited America in 1854 in order to study the effects of our climate upon agriculture. He asserts that the influences of moisture in tempering the sun's rays is a remark able fact and well worthy of further investiga tion. When the dew-point is high, or the air is filled with moisture, radiation from the earth is prevented and the temperature of the night remains almost as high as that of the day. When the dew-point is low, the sun's rays pass without absorption to the earth, and impart little of their heat directly t o the air. The medium dew-points are therefore most favorable to extreme heat in the atmosphere, and the greater heat beyond the tropics owing to this cause. The fact that the amount of moisture in the air regu lates the temperature of the nights has not received the attention it deserves. The great
amount of moisture in the air within the tropics is the cause of the warm and brilliant nights. Radiation from the air and ground, under these conditions, seems to lose its power. On the other hand, travelers in all parts of the world inform' us, incidentally, as to the connection between dry air and cold nights. Mr. Inglis, in his travels through Spain, relates that he was oppressed by the hot rays of the sun in the valley of Grenada while the hoar frost was lying white in the shade. Eastern travelers in the desert often complain of the broiling heat of the air during the day, and of its chill temperature at night. The means of warding against, the effects of frost by any covering, however slight, to prevent radiation, has long been known; also, by means of smothered fires, allowing the smoke to settle like a cloud over orchards and other ex posed situations. Frost, however, forms at night when there is sufficient air moving to carry away the smoke, and hence the means noticed are only clearly beneficial in a still atmosphere. This still atmosphere, however, is usually present during the exhibition of untimely frosts. Water begins to freeze when the temperature of the air is at 32° Fahr. At this temperature ice begins to appear, unless some circumstance, for example, the agitation of the water, prevents its formation. As the cold increases, the frost beconies more intense, and liquids which resist 'the degree of cold required to congeal water at length pass into the solid state. When water remains at complete rest, it may be cooled down to 28° Fahr, without freezing; but the moment it is agitated, the thermometer rises to 32° and the water freezes. In this case the insensible heat of the water is retained when the fluid is at rest. No experi ments have bitherto ascertained to What depth frost will extend, either in earth or water, but its effects will, of course, vary according to the degree of coldness in the air, the longer or shorter duration of the frost, the texture of the earth, the nature of the fluids with which the ground is impregnated, etc. In England the frost rarely extends in the ground below eighteen inches from the surface. In some portions of the United States it penetrates to the depth of several feet; yet in these localities many summerplants are raised in the open air, that in England must be kept under glass. During severe frost almost ail vegetables fall into a state of decay, and even a moderate degree of frost is sufficient to destroy many of the more tender kinds. The injury which vegetables sustain from frost is greatest when it is preceded by a thaw or copious rains; for the plants are then turgid with moisture, which, expanding in bulk as it passes into the solid state, produces the rupture of the vegetable fibers. Therefore it is that a sharp wind, or any thing which dries the sap or juices of vegetables previous to frost, tends to their preservation. The great power of frost on vegetables is well known. Trees are sometimes destroyed by it as if by fire, and split with a noise resembling the explosion of artillery, since the, juices of the tree expand with great force, as they are converted into ice. Frost, however, is most beneficial to the agriculturist in disintegrating heavy clayey soils, turned up to its influence in winter, and the rougher the plowing is left for the winter, the more beneficial its effects.