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Obs.-A bladder, tied in the same manner, will swell and burst, if laid before a fire; thereby proving the power of heat to rarefy air.

604. Common air may also be compressed, by cold or by mechanical means, into forty thousand times its ordinary space, and still maintain its elasticity; and on this principle is founded the invention of the airgun. Air has a constant disposition to maintain its equilibrium, level, or equal diffusion, like water.

Hence, if a bladder, filled with rarefied air, burst, an explosion takes place, from the rushing of the surrounding air to fill the space.

The same principle is the cause of all wind, which may be traced to some local expansion or compression of air by heat and cold ; thus, also, smide is carried up chimneys.

Obs.-- It is evident, that the density of bodies must be diminished by expansion ; and in the case of fluids and gases, the parts of which are mobile, many important phenomena depend upon this circumstance. If heat be applied to fluids or to gases, the heated parts change their places and rise ; and the colder parts descend and occupy their places. Currents are constantly produced in the ocean and in great bodies of water, in consequence of this effect. The heated water rises to the surface in the tropical climates, and flows towards colder ones; thus the warmth of the Gulf-stream is felt a thousand miles from its source, and deep currents pass from the colder to the warmer parts of the sea : the general tendency of these changes, is to equalize the temperature of the globe.

605. One of the principal foreign bodies mixed with, or dissolved in the atmosphere, is the vapour of water which is constantly rising at every degree of heat, provided the force of the vapour already in the atmosphere is not greater than that of vapour at the existing temperature. By this perspiration of the globe, 36 inches of wa

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ter per annum are raised from the surface of all seas or rivers ; and, at least, 30 inches from all land.

In December and January, it is 14 inches per month; and in July and August more than five inches.

606. By this constant process of evaporation, 100,000 cubic miles of water are, every year, raised into the atmosphere ; the greater part of which, at a certain height, parts with its heat, and is condensed into clouds.

These are carried by the winds over the land, broken and precipitated by the action of mountains and trees; and thus rendered the means of watering the soil.

It then returns to the sea in the currents of rivers; so that there is a constant circuit of the waters! They are chiefly raised from the sea, are carried by the winds over the land ; fall in rain ; and then return again to the sea in rivers ! The streams, their beds forsaking, upward move, And form again, in wand'ring clouds above : Hence, rich discerning showers; hence, balmy dews Their plenteous sweets o'er brightening fields diffuse ; Hence, shoots the grass; the garden smiles with flowers; And sportive gales steal fragrance from the bowers.

Obs.- In the process of evaporation the salt of the sea is not taken up: the water from the clouds is therefore fresh and pure.

607. The quantity of rain which falls in Great Britain is about 24 inches per annum in the eastern counties, and 36 in the western ; because these receive the first clouds as they are brought from the Atlantic by the westerly winds.

In the West Indies, 120 inches fall annually ; and in the East Indies, from 80 to 100.

As mountains are conductors of heat and electricity, and precipitate clouds, so it constantly rains on the Andes, and seldom rains in Siberia and Tartary ; the clouds generally falling before they reach those countries in their passage from the oceans.

Obs. 1.- This principle of the effect of high conductors on clouds, led to a public proposal, by Sir Richard Phillips, in 1793, for the erection of artificial conductors in Great Britain, and other civilized countries, by which the descent of rain might in various degrees be regulated.

2. Of course, as much rain falls as is evaporated; and it may be supposed, from the causes above named, that at least 75,000 solid miles of water fall every year on the land only, and the rest in the sea.

608. Springs, rivers, &c. are attributed to rain. Rain oozes down by the crannies of the stones, and. enters the caverns of the hills.

These being filled, the overplus water runs over by the lowest place, and breaking out by the sides of the hills, forms springs.

These running down the vallies, between the ridges of the hills, and uniting, form little rivulets or brooks; and these, meeting in one comraon valley, become a river, which runs into the sea,.

---the common level.
I see the rivers in their infant-beds !
Deep, deep I hear them, lab'ring to get free ;
I see the leaning strata, artful raug'd:
The fissures gaping to receive the rains,
The melting enows, and ever-dripping fogs.
Strew'd bibulous above, I see the sands;
The pebbly gravel next, the layers then
Of mingled moulds, of more retentive earths
The gutter'd rocks, and mazy-running clefts,
In pure effusion flow. United, thus
Thi exhaling sun, the vapour-burden'd air,
The gelid mountains, that to rain condens'd
These vapours in continual current draw,
And send them, o'er the fair-divided earth,
In bounteous rivers to the deep again;
A social commerce holding firm support:

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THOMSON

609. The instruments for making observations on the atmosphere, are the barometer; which ascertains the weight of the air, and varies in height between 30.1 and 281 inches.

The thermometer ; which ascertains the degree of

heat by the expansion or contraction of a fluid in the bulb, which sensibly affects the quantity in a small connected tube.

Thirty-two degrees are called the freezing point; and 212° is the heat of boiling water. It is hot weather "at 70; but it has been in England as high as 95 ; and is, in winter, sometimes 50 degrees below the freezing point, or 18 degrees below O, or zero. At 40 degrees below 0, mercury freezes.

Obs.-Mr. Wedgwood's clay-Pyrometer measures varia. sions of heat, as high as 32,000 degrees !

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At A there is a bulb or ball filled with quicksilver or spirits; which distending or shrinking with heat or cold, raises or falls the thread of the same liquid, contained in the connected tube ; and the scale and figures at the side, indicate the relative degrees of heat and cold.

For the purpose of acquiring a scale, the bulb is first plunged into melting ice, and the place where the liquid stands is marked; the bulb is afterwards plunged into boiling water, and the same operation repeated. On Fahreinheit's scale, this space is divided into 180 equal parts; and similar parts are taken above and below, for extending the scale ; and the freezing point of water is placed at 32 degrees, and the boiling point at 212 degrees.-1.8 degrees of Fabreinheit are equal to one degree of the Centigrade thermometer; and 2.25 to one degree of Reaumur.

T'he Barometer consists of a basin of mercury at the bottom, into which a tube, closed at top, and open at bottom, deprived of its air, is plunged ; the external atmosphere pressing then by its elasticity on the surface of the basin, raises the mercury in the tube to a height equal to the elastic force of the air ; varying between 28 and 31 inches, and indicated by an aceurate scale at the top.

610. Besides the above, there are hygrometers, to measure the moisture of the air; rain guages, to take the depth of rain; electrometers, to measure the electricity; and anemometers, to measure the velocity of the wind.

By these last, it appears, that wind is just perceptible when it moves two miles in an hour; that it is brisk, at 15 miles; high at 35 miles; blows a storm, at 50 miles; and a hurrieane, at 100 miles an hour: tearing up trees, and carrying away buildings.

In England, the wind blows twice, or nearly thrice 'as much from the west as from the east; and the wind from the south is to that from the north, in the proportion of 3 to 2.

Of what important use to human kind,
To what great ends subservient is the wind ?
Where'er th' aerial, active vapour fies,
It drives the clouds, and ventilates the skies :
Sweeps from the earth ipfection's noxious train;

And swells to wholesome rage the sluggish main.
611. The primary cause of all wind is the heat of
the sun ; which, during the diurnal rotation of the
earth, passes over some parallel between the tropics,
from east to west, every 24 hours.

Hence, as the air which is beneath the sun, is every way rarefied, there is a regular wind following the sun in the tropical parts of the world ; which, in some parts, is so regular and so unruffled, particularly in

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