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it furnishes a large supply of food, it renders distant places easily accessible, it purifies the air and regulates its temperature, and supplies the clouds with rain. The salt which is mingled with the water of the sea renders it more buoyant and less liable to freeze or to putrefy.

OF MOUNTAINS, CLOUDS, AND RIVERS.

Of the solid contents of the globe we know little, having but as it were scratched its surface. So far as has been

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observed, it consists of many distinct layers or strata of rocks of different kinds. These strata do not always lie in a horizontal position, but are in many places very much convulsed. Mountains are nothing more than elevations of these strata which seem to have been thrust up by some mighty force from beneath.

Stupendous as is the bulk of the mountain ranges which diversify the face of the earth, their size is as nothing in comparison with the bulk of the whole globe, being in the proportion of a grain of very fine sand to a common artificial globe.

Granite is the rock which forms the lowest known mass, yet it has not only upheaved the strata lying upon it, but has burst through them, and forms the summit of the lofty Alps and other mountains.

These irregularities of strata bring within the reach of man the materials that he applies to architectural purposes, and minerals, such as coal and iron, which would otherwise have been inaccessible.

By the action of the surrounding atmosphere, and the sun's rays upon the oceans and seas, large quantities of pure water (the salt and other ingredients being left behind) are taken up into the air in the form of vapour, which is invisible at first, but afterwards collecting in considerable masses, is suspended in the heavens, and constitutes the clouds that we see almost continually above us. The water of the clouds is not in the state of very minute drops, but of little vesicles hollow within.

When the moisture of the atmosphere is condensed by a cold wind, or the clouds are made to collapse by the discharge of their electricity, as in a thunder-storm, the vapour now formed into drops falls to the earth by its own weight in showers of rain, or if frozen, of snow or hail.

At the equator the heat is greatest, so that here the evaporation is greatest, and hence the quantity of rain is most abundant; the quantity diminishes as we approach the poles, but the number of rainy days increases; for whilst in hot climates the rain, when it does fall, comes down in torrents, as you approach the colder, the showers are less copious, and hence of longer continuance. All the water which is incessantly pouring into the Caspian and some other seas, is taken off by evaporation.

The water which has fallen on the ground, after moistening its surface, or penetrating the strata and again appearing as springs, seeks by its own weight the lowest level, and first in brooks, and then in rivulets and rivers, once more reaches the ocean: thus the waters of the globe are kept in constant circulation.

Mountains act an important part in the formation of rivers. They attract and condense the clouds, and thus feed the sources of the rivers with never-failing supplies. The loftiest mountains generally give rise to the largest rivers; thus the Andes give rise to the Amazons. You may easily ascertain the highest part of a country by observing where rivers having nearly a common origin run in opposite directions thus the Rhine and the Rhone indicate the Alps as the highest ground in Europe. When a river runs through a country that has a considerable descent, its course is rapid and generally direct, but when it traverses a flat country it pursues a slow and meandering course, its stream is divided by trifling obstacles, and if this should take place near the sea, it enters it by several mouths. Those rivers which overflow their banks receive their periodical excess of waters from the melting of the snows on the high grounds where they have their source, or from the immense quantities of rain which fall in tropical countries during the wet seasons.

OF THE ATMOSPHERE AND CLIMATE.

The atmosphere which surrounds the globe is about 45 miles high, and at the surface of the earth, which is in reality the bottom of a great aerial ocean, presses upon us

But its density The air is neces

at the rate of 15 pounds per square inch. diminishes very rapidly as you ascend. sary both to vegetable and to animal life; and it is air in motion, whether gentle or more impetuous, that constitutes wind-the zephyr or the hurricane. Air is so thin a substance that it is very easily disturbed; hence in many parts of the globe winds blow without any regularity, just as they are at the moment impelled by temporary and local causes. But over a large tract of the earth the currents of the atmosphere are constant; in this case, they are called trade-winds.

The trade-winds depend upon one of the two following causes :the unequal temperature of the equator and the poles, and the earth's motion upon its axis. When the door of a room in which a fire burns is opened, the cold air from without rushes in at the bottom, whilst the hot air, being lighter, makes its escape at the top. The same thing takes place on a large scale on the earth-the cold air of the polar regions finds its way to the equator, whilst the warm and light air of the equator supplies its place. The rush of cold air from the north is generally most impetuous in the spring, in consequence of the accumulation of a winter's cold; hence the storms experienced in this country at the time of the vernal equinox, generally from the north-east. The accumulation of a summer's heat in torrid climes causes a similar rush of air from the equator to the poles in autumn. When the northern current passes over our island, it seems to blow from east as well as north. The reason is, that whilst it is travelling due north and south we are moving from west to east. The two motions united give a wind from the north-east. When within 28 degrees of the equator, the increasing rapidity with which the surface of the globe moves near the line, aided by local causes, apparently annihilates the northern direction of the current, and the wind is nearly due east. It is here that the tradewinds are said to begin. From this point the wind gradually acquires the motion of the earth-it takes a northerly direction, and when it approaches the line it seems to blow due north. The currents from the South Pole obey the same laws.

A great diversity of temperature prevails over the globe. The greatest degree of natural cold ever observed is-58° (or 90° below the freezing point); this was in the Esquimaux country of North America; at Benares, on the other hand, the thermometer has been observed as

high as 120 degrees and upwards. In general the climate of the regions in the torrid zone is hottest, the temperature diminishing as we depart from it; because the sun's rays fall perpendicularly or nearly so on the torrid zone, but reach the countries that are on each side of it aslant, and so being spread over a larger surface cannot be so concentrated in any one spot.

But a variety of causes modify this general law. 1. The neighbourhood of the sea greatly moderates the cold of winter and the heat of summer. 2. The elevation of the ground greatly affects climate; thus, at an elevation of 15,000 feet above the sea you will have perpetual snow, even at the equator. 3. The length of the day, and the almost total absence of night during a part of the year in high latitudes, cause an accumulation of the sun's rays, which renders their short summers exceedingly hot. Their long winters are correspondingly cold. 4. The clearness of the sky in hot climates, if it fail to intercept the sun's rays by day, allows the warmth of the earth to escape by radiation at night: cold is by this means produced in some parts of India sufficient to freeze water. In Britain and other temperate regions, the clouds which commonly prevail intervene to prevent the warmth that we ill can spare from shooting off into the unbounded vault of the sky.

OF TIDES AND CURRENTS.

The ocean is subject to periodical elevations called tides. These are occasioned by the attraction of the moon and the sun.

At the same time that the earth by the principle of attraction draws the moon to it, the moon, in proportion to its bulk, draws the globe

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with all its contents out of its orbit towards itself, (as at c). That part of the surface of our orb which is nearest the moon will feel its attractive influence most, and be most powerfully drawn towards the moon; the central part will also be attracted, but in a less degree, and the most remote will experience it least of all. The moon exercising this unequal attraction upon the earth, will draw towards it, in the form of a huge wave, (as at F,) that part of the ocean which is directly opposed to it, whilst it leaves the ocean on the opposite side also in the form of a huge wave (highest in the centre) lagging as it were behind, (as at A. and E). Thus the approach of the moon and its removal both produce the same effect, the elevation of the waters above their natural level. To supply this accumulation of waters at these two opposite points, the parts between are deprived of their usual portion, and it is low water at these places. In the course of 24 hours, 50 minutes, each successive portion of the earth's circumference is presented to the direct influence of the moon, it will therefore be high water at each place twice in that period, or once in 12 hours 25 minutes. At new and full moon tides higher than usual, called spring-tides, take place. At these periods the sun and moon are in the same line, (c, м, S,) with respect to the earth, and the attractive force of both is combined in raising the waters of the ocean. At new moon the moon and sun are on the same side, but at full they are on opposite sides of the earth.

At intermediate periods between new and full moon the sun is at right angles with the moon, (as when the moon is at M,) and is using his attractive force to elevate where the moon's influence is diminishing the height of the waters. The lowest or neap tides take place at these periods. The ocean wave is most elevated at the equator, the point nearest the disturbing luminaries, hence the tides, when flowing, run from the line northward and southward to the poles. In the great oceans the water is not elevated more than a foot above the low water level, but in bays and confined channels much more; at Bristol the elevation is sometimes 50 feet, in the bay of Fundy 120 feet. Those seas that have little or no communication with the ocean have little or no tides.

There is besides, from the same cause that produces trade-winds, the difference of the temperature of the equator and poles, a constant circulation of the waters of the sea. A great current is observed to set from the poles north and south towards the equator, and preserves its direction for a long space. It is owing to this cause that floating masses of ice are frequently found in the temperate seas. Captain Parry attempted to reach the North Pole by traversing the intervening fields of ice by means of sledge-boats and rein-deer, but the southerly progress of the ice drifted by this current was so great as to render his

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