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hollow air-tight body a b c d is floating, having a tube, eƒ, opening into the interior. When placed into the water it is evident that a certain portion of the liquid will rise in the tube, and if light weights are added either below or above, the whole body may be caused to sink until its top is even with the surface of the fluid; ag is likewise a tube, which containing air, will prevent the instrument in great changes of weather from sinking to the bottom, bh, is a wire, and g h and i b are threads stretched obliquely from the tube to the wire. As the instrument rises and falls, a little bubble of air on the thread shows the motion; the threads are so located that when the bubble reaches i on the lower, it commences at h on the upper one. A change of the pressure of the atmosphere preceding a wind or rain, by reason of a diminished pressure upon the surface of the water in the vessel, A B C D, forces less of the fluid into the tube ef, and thus the specific gravity of the included air being lessened by its expansion, the instrument rises, and the bubble descends, corresponding with the fall of the mercury in the barometer. When, on the approach of fine weather, the atmosphere becomes calm, and of its usual density and elevation, the water being forced into the tube ef, by the increased pressure causes the baroscope to sink, and consequently the bubble to rise. It is said that this instrument will show alterations in the air 1200 times more accurately than the common barometer. The inventor, Mr. Caswell of Oxford, observes that the bubble is seldom known to stand still even for a minute; that a small blast of wind that cannot be heard in a chamber will sensibly make it sink, and that a cloud passing over it always makes it descend. The greatest objection to this very simple instrument is, it is liable to be affected by the expansion of the included air by heat.

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"He spoke, and, at his call, a mighty Wind,
Not like the fitful blast, with fury blind,
But deep, majestic, in its destined course,
Sprung with unerring, unrelenting force,

From the bright East. Tides duly ebbed and flowed;
Stars rose and set; and new horizons glowed;
Yet still it blew."

Rogers.

In the preceding chapter we have alluded to the weight and decreasing density of the air. In the present we shall consider it in its relations to heat and moisture, in which connection it performs a most important part in the economy of nature. Air, like all material bodies, expands when heated, and thus its specific gravity being lessened, it rises. Every one is familiar with this fact. The little whirligigs placed on stoves, are turned by the ascent of the heated air, and the draft of chimneys and pipes depends upon the same principle. We might infer that if air expands upon being heated, it will part with this heat on being condensed. This is the fact, upon forcing a close fitting piston down a tight tube, suddenly compressing the air before it, a heat is evolved, sufficient to inflame good tinder. We well remember in our younger days performing this feat, using a leaden tube for the pipe, and a bit of wood with a circular and greased leather nailed to one end, for a piston, placing the "punk" in the folds of the leather.

The ascent of heated air from the earth is one of those silent and often unobserved agencies employed to produce a general equilibrium in the temperature and moisture of the earth. The air itself being transparent, is scarcely heated by the passage of the suns rays, but the stratum nearest the earth absorbing a portion of the heat, rises and gives place to a colder portion, which in its turn is displaced by another, and thus the excessive heat which, in some portions of our globe, would parch up the earth and destroy all life, is rapidly carried off, and a more genial temp

perature produced. The heated portions of air distribute their warmth throughout the great body of the atmosphere, and sometimes in the form of winds sweeping over colder regions of the earth, moderate the rigor of the climate.

Ascending from the earth we find the temperature of the air constantly diminishes until we arrive at a region of frost, the limit of which, is called the term of perpetual congelation. The height of the term of congelation varies for every change of latitude. In the following diagram taking A B, for the height of

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B 0° 10° 200 302 40° 500 60° 70°

800 C

perpetual congelation at the equator, and B C, for the line of latitudes, then the decrease in the height of the term of congelation from the equator to the poles, may be represented by the several parallels bounded by the curved line A C. The region of perpetual frost at the equator is at a height of 3 miles, at a distance of 35° from the equator, about 2 miles, at 54° about 1 mile, at 80° very near the surface of the earth, and at 90° the surface of the earth. It will be well to fix in the mind, the limits here assigned, as we shall often refer to this varying elevation in explaining the phenomena of rain, hail and snow. The clouds almost always float below the term of congelation.

Although the temperature of the surrounding atmosphere is

TEMPERATURE OF, VALLEYS.

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generally colder as we ascend, yet from local circumstances the heat of the earth being increased, the air becomes sometimes very mild even in elevated districts. Valleys, it is well known, are warmer than level plains in the same latitude on account of the reflection of the sun's heat from neighboring hills and mountains. In Switzerland, instances occur of spots of verdure in the midst of perpetual snow and glaciers. We give below a view of the glacier of Grindelwald in the Canton of Berne. Here woods and meadows border close upon the immense fields of ice, which

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descending from the upper regions, cover an extent of about 1200 square miles of territory. The ice is seen presenting innumerable peaks in the gorge between the mountains. It is said that there are plains in the Himalayah mountains 15,000 feet above the level of the sea which produce fine pasturages.

Air when put in motion produces what is called wind, and the variable distribution of heat throughout the atmosphere is the main cause of wind, or flow of the air, for thus the local density is constantly affected, and the equilibrium of the mass disturbed. Winds are exhibited in various forms, breezes, high winds, gales, hurricanes and tornadoes. These varieties depend chiefly upon their different velocities, a velocity of twelve miles an hour making a strong breeze; sixty miles, a high wind; one hundred miles a hurricane.

The force of the wind when moving with the velocity of a hurricane or tornado, is almost incredible; when we speak of the geological changes that have passed over the face of our globe

during its past existence, we shall have occasion to refer to their agency; no power can resist the combined action of winds and waves. Hurricanes and high winds are generally characterized by a whirling motion producing the phenomena called whirlwinds, these often exhibit the most incredible power, uprooting huge trees, and whirling their dismembered fragments into the air, unroofing houses, and even raising aloft animals, and heavy carts. The great gales of the ocean manifestly exhibit more or less of this rotary motion. Such are the the hurricanes which annually sweep over the Indian seas and along the Atlantic coast.

Our limits will not permit us to be very extended upon this subject, and we shall therefore briefly notice some of the more peculiar winds, and we commence with the land and sea breezes. These are occasionally met with in every latitude, but are constantly observed near the shores of the continent, and of the larger islands within the tropics. In these sultry regions, as the day advances, a refreshing wind blows from the sea, and is succeeded by an opposite current from the interior of the land on the approach of evening. The cause of these diurnal winds is obvious. The change of temperature between the night and day on land often varies more than 40° or 50° Fahr., while at the same time on the water it seldom varies more than 10 or 20. The large body of heated air over the land, rising continually during the day, a denser and colder portion rushes in from over the water to supply its place, causing the sea breeze. During the night the ground cools much more rapidly than the water, and the lower stratum of atmosphere thus soon becomes colder than at sea, consequently a stream of air thus flows toward the sea, displacing the lighter and warmer air, producing the land breeze. This breeze is never as powerful as the sea breeze, but is much colder. Every one who is familiar with these breezes must have noticed the period of peculiar languor and depression, between the change from the sea to the land breeze.

Dr. Robinson mentions an experiment which illustrates the cause of land and sea breezes very prettily. If we place a hot stone in a room, and hold near it a candle just extinguished, we will see the smoke move toward the stone, and then ascend up

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