Hoar-frost-the ice of dew and mist-is formed by the temperature of the atmosphere falling below 32°, the freezing point; or by the powerful radiation of heat from the substances receiving moisture depressing their temperature to the point of refrigeration, in which case the particles freeze, even while a thermometer may be several degrees above 32°. Nature never appears more beautiful than on the mornings of a strong hoar-frost, when the fogs have vanished, and the bright cerulean of the sky is expanded over the productions of the earth, which, from a state of nakedness, have put on vestal garments of the most exquisite purity and delicate finish, the suddenness of the change giving it the aspect of a work of enchantment. Every part of the vegetable kingdom, from the humble blade of grass to the gnarled and majestic oak of the forest, has acquired a character of mysterious loveliness, which surpasses any idea of the scenes of fairy-land, and affects the mind by the silence, rapidity, and extent of the creation, as well as by the consciousness of its speedy departure. In the woods, the dark boles of the trees render more impressive their silvery crests, from which the rime descends in snowy showers as the birds twitter among their branches. The forms assumed by the ice of dew are classed by Mr. Howard into the spicular and granular. The former are very minute icicles which appear upon fibrous surfaces; the latter are globules frozen as they hang pendant from the substances upon which moisture has been deposited. But in whatever solid form the vapours of the atmosphere are returned to their native terrestrial home, their reduction to a liquid state speedily follows, except towards the poles, and on high mountain elevations. The snow of the fields, the hoar-frost, and the ice of the rivers, dissolve under a change of temperature, yet in obedience to a very peculiar law which ensures their gradual retirement. The moment the change arrives at a particular and invariable degree of heat, thaw commences; but any further addition of heat is absorbed or rendered latent, and the temperature of the thawing mass remains stationary till the dissolving operation is complete. But for this circumstance, which has been called a violation of law, the operation would be instantaneous, and productive of the most disastrous effects. Under the first touch of the warmth necessary to ensure a thaw, the magical spectacle of hoar-frost would be gone in a moment. The snow would rush from the fields in a resistless inundation to the rivers, and the frost-bound streams would be relieved of their ice before the skater could reach their banks.

CHAPTER XV.

PHYSICAL CLIMATE.

E use the word climate to denote the temperature of the air in various regions of the globe-an all-powerful cause in the determination of their physical aspect. Great diversities meet us here, from the extreme of cold, which produces perpetual congelation towards the poles, where no vegetable life is found, to the fervid heat of an equatorial district, under the action of which, in connection with the prevailing moisture, vegetation attains its greatest luxuriance. It has been remarked that it is easy to conceive of an astronomical arrangement, according to which all parts of the earth would have had the same,

or nearly the same, climate. But, had this been the case, our planet, though full of life, would not have been furnished with that useful and agreeable variety of vegetable productions and animal forms which it now presents, and many articles of convenience and luxury which the existing arrangement affords to the human race would have been wanting. It is an instance of wise and benign adaptation that the human frame has been so constituted as to be able to bear the striking diversities of climate which distinguish the abode of mankind, both the heat of the burning plains of Beloochistan, and the cold of the icy shores of Greenland. It has been proved by experiment that the body is capable of enduring very great extremes of temperature, and sudden changes of it, without being seriously affected. Dr. Fordyce exposed himself to an atmosphere raised to the temperature of 200° of Fahrenheit, or nearly to the boiling point of water, for ten minutes, and a thermometer fixed under his tongue indicated only 98°, so that his body resisted the impression of the artificial heat, and retained nearly its natural temperature, which the surrounding air exceeded by more than 100°. On the other hand, a degree of cold which depressed the mercury in a thermometer 52° below the freezing point, has been borne with only a slight addition to the ordinary clothing. Owing to this capacity of the frame, man can accommodate himself to the circumstances of the earth he inhabits, live amid the perpetual ices of the north, or under the fierceness of a torrid climate, and pass without difficulty from the one to the other, for the purposes of commerce and enterprise. Though many members of the vegetable kingdom show a marked indifference to change of temperature, and hence are transportable with success from cold to heat, or from heat to cold; yet vast numbers exhibit as marked an incapacity for the transition. The cinnamon bushes that clothe the surface of Ceylon would not endure a removal to the bleak moors of England, nor our highland pines a transplantation to the level plains of Hindostan; but the different members of the human family, in varying degrees, display a physical adaptation for emigration into any of the habitable parts of the earth, however discordant their temperature, the frost biting in one region, the fire-king breathing in another.

We proceed to mention the causes which determine physical climate, and induce its differences:

1. The geographical position of a country with reference to the equator is one of the leading circumstances by which its temperature is determined. At the equator, and within the tropics, the greatest heat is experienced, because the sun is always vertical to some place within those limits, and the solar action is the more intense in proportion as the rays are perpendicular to the earth. As we recede from the equator, north or south, their direction becomes more oblique, and less influential in promoting temperature. It is for this reason that a declivity towards the equator, which receives the solar rays more directly than a level surface, is always the warmest, and hence the importance attached to an inclination of the surface with a southern aspect in our climate. The latitude of a place is therefore a prime determining cause of the temperature to which it is subject, a decrease of heat taking place with an increase of distance from the equatorial localities, though with various modifications, which will be hereafter noticed. One exception taken to this general rule may here be stated. It is true of countries lying between the tropics and the poles, that heat decreases with an increase of latitude, but it is thought not to be true of countries between the tropics and the equator. Tracing the path of the sun as delineated upon an artificial globe, we perceive an advance of 12° of latitude made in the first month after the equinox, only 8° in the second, and but 31° in the third; and upon retiring from the tropic to the equator, he follows the same course inversely, traversing 31° of latitude in the first month, 8° in the second, and 12° in the third. It follows, therefore, that to all places situated within 3° of each tropic, his rays at noon are vertical, or make but a slight angle for two months respectively; whereas those places which are under the equator have but the sun as near their zenith for about a week at each equinox. From this circumstance it has been inferred that in receding from the equator there is no decrement in the mean annual temperature till we have passed 2340, the latitude of the tropics. This cannot yet be regarded as an established fact, though it is quite certain that the mean temperature in summer near the tropics is higher than near the equator. In the northern hemisphere the countries where the greatest heat is experienced-the banks of the Senegal, the Tehama of Arabia, and Mekran in Beloochistan - coincide with the tropic of Cancer; and it has been observed that the snow-line of the Andes in 17°

south latitude is higher than at the equator, an evidence of a higher temperature. With this exception, if admitted to be one, the decrement of heat proceeds gradually as we travel along the same level from the line to the poles.

2. The temperature of countries is largely affected by the extent of their elevation above the level of the sea. It is well known that as we ascend in the atmosphere the cold increases, -an effect due to the rarefaction of the air, and to the circumstance of being further from the heat reflected from the surface of the earth. We must travel several hundred miles along the surface from the equator, before we become sensible of a diminished temperature; but an ascent there of only a thousand yards will sink the thermometer 10°, or 1° for 310 feet, and the decrease of heat will go on as we ascend higher, with a little irregularity, till at the height of somewhat less than 16,000 feet, or about 3 miles, we come to the line of perpetual congelation. The ratio of the diminution of temperature usually given, is 1° for 300 feet of altitude; 2° for 595 feet; 3° for 872 feet; 4° for 1124 feet; 5° for 1347 feet; and 6° for 1539 feet. In the temperate zone generally, if one site is a thousand yards higher than another adjoining, it will have a climate 12° colder; and the higher the latitude the lower the snow-line becomes, till it osculates with the surface of the earth in the frigid zone. The following diagram

represents the line of perpetual snow, forming the arc of an ellipsoid, passing over the equator, from pole to pole.

But this important boundary-line, as it exists in nature, exhibits no continuous curvature, though attaining a considerable elevation at the equator, and level with the surface in high latitudes. It repeatedly oscillates, suddenly ascends and descends, under control of the physical peculiarities of different regions. The snow-line of the Andes, at the equator, is found at the height of 15,748 feet; but not a particle of permanent snow was seen by Mr Pentland at the following sites on the Bolivian part of the chain, though in from 17° to 19° of south latitude :

A remarkably great and sudden fall of the Andean snow-line occurs further south, unparalleled in any other part of the globe.

Thus, within the distance of only 10° of latitude, the snow line experiences a difference of elevation of 9000 feet, an effect attributed to the fact of Chiloe being covered with forest trees dripping with moisture, indicating a clouded sky and little heat in summer, while in Central Chili rain does not fall for the seven summer months; the sky is generally clear, and the climate hotter. The subjoined table gives the height of the curve of congelation in different latitudes, exclusively from calculation founded upon the known law of the decrease of heat by elevation.

These results are valuable in the light of general approximations, but they are not to be regarded as rigidly applying to all places in the respective latitudes stated. The elevation of the snow-line under the same parallel exhibits irregularities which are the effect of local circumstances. Thus there are plains on the Himalaya mountains 15,000 feet above the level of the sea, which produce fine pasturage; and barley and buck-wheat flourish at the height of 11,000 feet, which is above the region of perpetual snow on the Andes, in the same latitude. The southern declivity of the Himalaya, also, has the snow-line at an elevation of about 13,000 feet; but instead of descending as we proceed farther north, it rises to the height of 16,000 feet. This is clearly an irregularity