earth, like the brazen meridian of a globe, every part of the Equator, containing 360°, must have passed under that meridian in 24 hours. Hence, if we divide 360 by 24, we shall find how many degrees pass under this meridian in one hour, which will be 15. Now, as the earth really revolves in a direction from West to East, it is continually, as it were, falling below the heavenly bodies on the Eastern, and rising above those on the Western horizon; hence, those bodies appear to rise in the East, and set in the West, because they come into sight as the Eastern edge of our horizon falls below them, and are hid from view as the Western edge rises above them. If the sun, therefore, appears to rise at six o'clock at London, it will be yet an hour before it appears to rise at a place 15° West of London; two hours before it appears to rise at a place 30° West of it, and so on. In like manner, it will have already appeared to have risen one hour at a place 15° East of London, two hours at a place 30° East, and so on. Therefore, when it is twelve o'clock at noon at London, it is one o'clock in the afternoon at all places 15° East of London; and only eleven o'clock in the forenoon at all places 15° West of London. The following questions may serve to exercise the young student :

It is eleven o'clock in the morning at Vienna (Pl. III.)— Where is it noon, where is it one in the afternoon, and where is it ten in the morning?

When it is mid-day at London-Where is it midnight?

When it is two o'clock at Kingston in Jamaica (Pl. XXVI.) -What o'clock is it at Shrewsbury? (Pl. IV.)

Suppose an eclipse of the sun takes place at three in the afternoon at the place where I am, and I see by the almanack that it took place at half-past twelve in London -In what longitude am I?

CHAPTER II.

PHYSICAL GEOGRAPHY.

M. G. Plate II.

PHYSICAL geography is that science which treats of the nature and causes of the phenomena attendant upon our globe, and in its largest sense would include a far wider range than can be embraced in a work like the present. A slight sketch is, however, subjoined of such of these phenomena as relate to the distribution of heat, the prevailing winds and ocean currents,the amount of rain, the influence of climate upon vegetation, and the distribution of the various races of mankind. The most important of these subjects, and that which exerts the greatest influence on all the rest, is that relating to the distribution of heat.

If a flat surface be exposed perpendicularly to the fire, it will rapidly become heated; but if it be held in an oblique position, a great portion of the heat will be reflected, which would otherwise be absorbed. In like manner, if the rays of the sun fall perpendicularly on our atmosphere, and, through it, upon the earth, both the atmosphere and the earth will become much more heated than if the rays fall in an oblique direction. Those parts, therefore, of the earth which lie near the equator, and over which the sun is perpendicular, will be much hotter than those which are more remote, and upon which, owing to the earth's form, its rays fall obliquely; and

towards the poles, where the obliquity is very great, the greater part of the heat will be reflected, and only a small portion of it absorbed. We find, therefore, that the climate toward the equator is hot, and that toward the poles is very cold.

If this was the only cause which influenced the temperature, the Isothermal lines, or lines of equal temperature (Pl. II.), would exactly coincide with the parallels of latitude, and decrease in uniform succession from the equator to the poles. This, however, is found not to be the case. The equator of heat, or the line passing through those places which are hottest, is found as far as 10° N. of the equator in South America, and even 14° N. in Abyssinia and Hindostan; and the point at which the soil is permanently frozen at a small depth below the surface, is not more than 51° N. in Labrador; whereas London, which is about the same latitude, enjoys a genial climate; and the point of perpetually frozen subsoil is not found much below latitude 68°, in Finland: we must look, therefore, to some modifying causes.

The distribution of land and water has a considerable influence on climate: great masses of water are much less susceptible of a change of temperature than the land. The sea is therefore commonly warmer than the air in winter, and colder in summer. Insular positions, and those exposed to prevalent sea breezes, have a more equable temperature than those far inland, because the warmth of the atmosphere is modified by that of the water, over which it passes before it reaches the shore.

Much effect is due to the elevation of particular localities above the sea level. The atmosphere, which is supposed to extend in a very attenuated form to about the altitude of forty-five miles, is capable of retaining much more heat when compressed by the weight of the superincumbent column, than when less compressed. The higher, therefore, we ascend,

the less the height and weight of air above us, and the rarer the atmosphere, which is, therefore, able to retain less heat.

This decrease of temperature is very rapid, insomuch that perpetual snow is found at the altitude of about 15,000 feet at the equator, and at lower elevations as we proceed, north or south. The hottest mean annual temperature is about 81° of Fahr., and as perpetual snow implies a mean temperature not above 32°, 15,000 feet of altitude are found to be sufficient to reduce the temperature as much as 50° or 60° of latitude. This will explain how the plain of Quito, which is nearly 8000 feet above the sea, and some other points near the equator, have a less temperature than others further north or south which have a lower elevation.

Another point of some influence is the vicinity or absence of any large extent of snowy mountains. Ranges like the Andes, Himalaya mountains, or even the Alps, cool the air in their vicinity, and send down refreshing breezes to the hotter countries at their bases. Other modifying causes are the prevalence of certain winds, and of certain currents in the

ocean.

Air, like other bodies, expands by heat, and thus becomes of less specific gravity; it will therefore, when heated, ascend, and the cooler air will rush in to supply the vacant space. We might expect, therefore, that there should be a continually ascending current of air from the hottest part of the globe, and that the cold air from the poles would rush in, forming a north wind in the northern, and a south wind in the southern hemisphere. While, however, the cold air is rushing from north and south, the earth is revolving on its axis from west to east; and though it is true that the atmosphere revolves with the earth, and therefore the direction of the current might be thought to be uninfluenced by the earth's revolution, yet it is to be remembered that the atmosphere near the poles, say at lat. 80° for instance, revolves only through a circumference of about 2000 miles

in 24 hours; while at the equator, which is so much further removed from the axis, it revolves through a space not less than 25,000 miles in the same period. In passing, therefore, from the poles to the equator, the air has not acquired the velocity of the earth's surface, and is continually left behind. That is, toward the equator, the earth revolves faster than the north and south currents, which, therefore, appear to meet it, and become north-east and south-east currents. These are the trade winds.

The ascending current from the equator becomes rapidly cooled in the higher regions of the air, and gradually descends as it passes northward and southward, to supply the air subtracted from the poles. But having acquired the velocity of the earth at the equator, its rotation is more rapid than that of the earth in the temperate and cold regions. The upper current, therefore, in the northern hemisphere, is not a south, but a south-west wind, and in the southern hemisphere, north-west. This current descends to the earth between lat. 23° and 28°, from whence south-west winds begin to prevail in the northern, and north-west winds in the southern, temperate zone. The winds, however, in these portions of the globe, are very variable, not only from the alternate prevalence of the upper or lower current, but also from local causes.

The trade winds from this view of the case ought to blow most strongly and most duly from the west, at the point of highest temperature. It is found, however, that between the north-west and south-west winds there is an interval of about 5° or 6° of almost perfect calm, interrupted by sudden squalls of variable winds accompanied by floods of rain. The cause of this does not seem very clearly ascertained, but it may perhaps be due to the ascending current diverting the course of the streams of air from either side, and carrying them upward with it. Lesser variations in the force and direction of the trade winds are attributable to the distribution of land and