NATURE AND EXTENT.-Atmosphere is the term applied to that body of gaseous matter which surrounds the earth. It has been not inaptly compared to the down on the skin of a peach.

The air or atmosphere is composed chiefly of two gases, one-fifth being oxygen and four-fifths nitrogen. Mixed with these, but in very small quantities, are other substances, of which the most important is aqueous vapour, or vapour of water.

The air is very elastic, and is kept close to the earth by the attraction of gravitation. It is therefore most dense or heavy at the sea-level, and lighter in the higher regions; and this notwithstanding the greater heat of the lower strata, which tends to expansion. The compression of the air may be explained by comparing it to high piles of wool or hay. The lower portions being pressed by the upper, will be denser, that is, more compact, containing a greater weight in proportion to the space occupied than the upper portion, which sustains little or no pressure.

At the sea-level, the pressure of common air is equal to that of about 30 inches of mercury, or nearly 15 pounds on every square inch of surface. At the height of 12,000 feet it is only half this density. The

density of the air is almost imperceptible beyond 50 miles, but it may extend 200, gradually thinning away into the ether that occupies all space. Winds, dew, clouds, rain, and other matters that affect climate, are all formed within a short distance from the earth.

Of the changes to which the atmosphere is liable, two are important and easily understood, viz., changes of temperature and changes of pressure; that is, heat and cold, density and lightness. Temperature is measured by the thermometer (thermos, heat, meter, measure), in which the mercury expands and rises with the heat, but sinks, being contracted (con, together, tractus, drawn), with the cold. Pressure is measured by the barometer (baros, weight), which has a column of mercury, with no air at the top, balancing the weight of a column of air, the whole height of the atmosphere. If there is much vapour in the air, the pressure is diminished, for moist air is lighter than' dry, and rain may be expected.

USES. The atmosphere is the medium through which the sun's rays of light and heat are conveyed to the earth, and dispersed over its surface. It is likewise the great laboratory into which the waters of the earth are drawn up in the form of vapour, to be condensed and poured forth again in showers. The oxygen which the air contains is the aliment of fire, and indispensable for the support of animal life.

WINDS.-Winds are currents in the atmosphere, produced by one portion being denser than another. Whenever any portion of air becomes heated, it expands, becomes lighter, and ascends; while the colder

air around rushes in with more or less force to take its place.

A current of air proceeding at the rate of seven miles an hour is considered a gentle wind; at fourteen it is a light breeze; at forty-one a gale; at sixty-one a storm; at ninety-two a hurricane.

CONSTANT WINDS.-The land and sea breezes which occur daily in warm situations on the coast are the simplest and most regular of all winds. The land becomes heated more readily than the sea, and also cools more quickly. By about nine o'clock, therefore, every morning, in warm latitudes, the land has become hotter than the sea, and the air over it ascends to the higher regions, while the sea air pours in slowly at first, and then moré steadily. After sunset the land cools rapidly, and the process is reversed.

The TRADE WINDS exhibit this on a larger scale. The heated air of the torrid zone rises and flows over towards the poles, while the cold air advances to replace it. This cold current would be north and south if the earth were at rest; but it is carried, or rather it falls behind, towards the west, because the rotation of the earth from west to east is more rapid at the equator than in the regions from which the wind came, and it has not fallen in with the greater velocity. Therefore winds from the north-east blow steadily in the Atlantic Ocean and some parts of the Pacific throughout the year. From their influence on navigation they are called trade winds. In the Atlantic they blow on both sides of the equator, and there is a belt or zone of calms very near the line, and north of

it, owing to the preponderance of land in the northern hemisphere.

The current of wind coming from the north pole diverges to the west as it enters the torrid zone. The south current turns to the west in the same manner. On the other hand, the upper current of warm air which rises from the equator with the equatorial velocity of 1000 miles an hour, outstrips the speed of the temperate zone, and becomes a west and a north-west wind in the southern hemisphere, but a west and south-west in the northern.

PERIODIC WINDS. The regularity of the equatorial wind is disturbed by the arrangement of the land. There being much more land on the north side of the equator than the south, and much more in the eastern hemisphere than the western, causes the air to be unequally heated throughout the torrid zone, and interferes with the tendency to form one regular Also the existence within the tropics of lofty mountain chains, covered with snow, has an important influence on the currents of hot air rising from the plains and valleys. In the Indian Ocean, and some other places, the equatorial winds are periodic, and are called monsoons, blowing from one quarter for half the year, and from the opposite the other half.

current.

On the extensive level plains of the torrid zone, both in Africa and America, there are regular and constant east winds; but, being on land, they are not called trade winds.

VARIABLE WINDS.-With the exception of the land and sea breezes, the trade winds, and the monsoons, winds are more or less changeful and variable. Doubtless they obey the general law, but its operation is not so well understood.

CYCLONES.-There are some violent winds limited to certain districts. Such are those spiral storms, familiarly known as whirlwinds among us; hurricanes · in the West Indies, and typhoons in the Indian Ocean and China Seas. They are now generally all named, in scientific language, cyclones (cycle, a circle). They seem to arise chiefly from the meeting of contrary winds, though sometimes by obstruction of the land. These storms, though violent and destructive, are of short duration.

HOT WINDS.-The simoom of the Arabian desert is an abrupt and burning gust from the south, darkening the sky, blowing as if from some enormous oven, and often proving fatal both to man and beast. The hot wind which visits Turkey is called the samieli; in Egypt it is the khamsin; in Guinea and Senegambia, harmattan.

The sirocco is a hot wind originating in the burning deserts of Africa, and occasionally reaching Sicily and the adjacent parts. The solano, which sometimes visits the Spanish peninsula, is considered a modification of the sirocco.

AMERICAN WINDS.-Pampero is the name given in Buenos Ayres to a violent west wind which carries clouds of dust to the Atlantic coast. The puna winds from the snowy peaks of the Cordilleras, are so pierc