All cooling is occasioned either by the heat being conducted from a body by a colder, which is in contact with, it, or by radiating from the body cooled, though circumstances accelerate or retard the radiation ; and whatever checks the radiation of heat from a body is a screen, and keeps it warmer.

For example, a thermometer, placed upon a grass plot, exposed to a clear sky, fell to 35° ; but another thermometer, within a few yards of the preceding, but with the radiation of the rays of heat from the grass checked by no other covering than a cambric pocket handkerchief, declined no lower than 42°. No difference of result occurs whether the radiating surface be parallel or perpendicular to the horizon ; for when the mercury in a thermometer, hung against an openly exposed wall, fell to 38°, another thermometer, against the same wall, but beneath a web of gauze stretched tightly, at a few inches distance, indicated a temperature of 43°.

These results explain the beneficial operation of apparently such slight screens to our wall-fruit when in blossom. A sheet of canvas or of netting prevents the direct radiation of heat from the wall ; the cooling goes on more slowly, and is not reduced to that of the exterior air at night, before the return of day begins to re-elevate the external temperature.

The colder the body surrounding another body, the more rapid the radiation from the latter ; for it is a law of heat that it has a constant tendency to be diffused equally; and the greater the diversity of temperature between , two bodies in contact with each other, the greater is the rapidity with which the progress towards equilibrium goes on. This is one reason why a temperature of 32°, with a brisk wind attending it, will injure plants to a far greater extent than a temperature many degrees lower, with a still atmosphere ; but it is aided by the operation of another law of heat, viz., that aeriform bodies convey it from a cooling body, as a wall or a tree, by an actual change in the situation of their own particles. That portion of the air which is nearest to the cooling body is expanded, and becoming specifically lighter, ascends, and is replaced by a colder portion. This, in its, turn, becomes heated and dilated, and gives place to another colder portion. And thus the process goes on, until the cooling body is reduced to the same temperature as the air.

In a still atmosphere, this goes on slowly; the air in contact with the wall and tree rises very gradually as it imbibes warmth from them ; but if there be a brisk wind, a constant current of air at the lowest temperature then occurring, is brought in constant contact with them, and the cooling is rapid, in accordance with the law of equilibrium just noticed. A shelter of netting, or even the sprays of evergreens, are of the greatest service in preventing the sweeping contact of cold air at such limes. Snow is a good shelter; it prevents heat radiating from plants; protects them from the chilling blasts ; and is one of the worst conductors of heat. I have never known the surface of the earth, below a covering of snow, colder than 32°, even when the temperature of the air above has been 28°.

Strange as it may appear, yet it is nevertheless true, that a screen is more beneficial in preserving the temperature of trees, when from three to six inches from them, than when in immediate contact with their surfaces. When a woollen net was suspended four inches from the wall on which a peach tree was trained, the thermometer fell very slowly, and the lowest degree it reached was 38°; when the same screen was twelve inches off, it fell to 34°; and when drawn tightly over the tree, it barely kept above 32°, the temperature of the exterior air. When at twelve inches from the wall, it permitted the too free circulation of the air; and when in immediate contact with the polished bark of the peach, perhaps another law of cooling came into operation. The law is, that polished surfaces radiate heat slowest. Thus, if two glass bottles, equal in size and thickness of glass, and of the same shape, be filled with warm water, and one of the bottles be covered with an envelope of fine muslin, this bottle will give out heat to the surrounding air with much greater rapidity than the other bottle; so that, in a given time, the bottle with the envelope will be found colder than the one which has no covering.

Screens, such as the preceding, or the slighter agents, sprays of evergreens, placed before the branches of wall-trees or other plants, as already noticed, operate beneficially in another way, checking the rapid passage of the air over them: such passage is detrimental in proportion to its rapidity, for the more rapid it is, the greater is the amount of evaporation, and, consequently, of cold produced. Mr. Daniell says, that "a surface which exhales one hundred parts of moisture when the air is calm, exhales one hundred and twenty-five parts when exposed to a moderate breeze, and one hundred and fifty parts when the wind is high".

During all high winds, but especially when blowing from points varying between the east and the south, for they are the driest in this country, the gardener will always find shelter is beneficial to his plants, whether in blossom, or with fruit in its first stages of growth, for these winds cause an evaporation much exceeding in amount the supply of moisture afforded by the roots.

In March, such shelters are much required, for the winds are then violent and dry even to a proverb; but it is during the days of its successor, April, that sets in the only periodical wind known in this island. It comes inter-mittingly, and with a variable force from points ranging from east to northeast, and is one of the most blighting winds we have. It continues until about the end of the second week in May, though often until its close; and it is a good plan to have the tree, during the whole period, by day as well as by night, protected. This periodical wind is occasioned, probably, by Sweden and Norway remaining covered with snow, whilst England is some 20°, or more, warmer; and an upper current of warm air is consequently flowing hence to those countries, whilst a cold under current is rushing hither to supply its place. This wind, and its consequent cold weather, is so regular in its appearance, that in Hampshire, and some other parts of England, the peasantry speak of it as the black thorn winter, that bush being in blossom during a part of its continuance. - Princ. of Gard.

Not only are screens required for out-door plants, but for those under glass; and Mr. Paxton is quite right in saying, that "one of the things which should be constantly borne in mind, and more especially in the forcing season, is the most effectual means of keeping up the requisite temperature in the hot-houses with least fuel; and that, in all cases where practicable, the use of external coverings, if properly used, will render strong fires in a great measure unnecessary. Some coverings are used at Chatsworth constantly at night, which makes from ten to fifteen degrees difference in the temperatures of the houses where they are applied, and to maintain which, without them, would consume three times the quantity of fuel now necessary." - Gard. Chron.

For wall-trees, now that glass is become so much cheaper, the best of all screens may be employed, viz., glazed frames, of a length extending from the coping of the wall, to the surface of the soil, about two feet from the stems of the trees.