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CLASS IX....LESSON V.

OF AIR.

2. WHAT are the properties of air? A. It was formerly a received notion that all nature was composed of four elements, air, earth, fire and water; these ideas are now exploded, as they are all found to be compound substances; the air enters into the composition of all bodies, and exists in them under a solid form, in this state it is called fixed air; it is figuratively described as the cement of all other bodies.

As the air is a compound body, are the substances of which it is composed known?

A. Yes, chemistry which has unfolded to human curiosity a vast creation before unknown, has determined that it is composed of 22 parts of oxygen gas, or pure air, a true element, and 78 parts of nitrogen or azote.

2. What are these substances called oxygen and nitrogen?

A. The perfect knowlege of them is to be obtained only by considerable attention to chemistry, but in a brief way they may be thus described, oxygen supports flame and animal life, but oxygen alone would be too active for animal existence; the glorious author of nature has tempered it by the admixture of nitrogen, which appears to act upon it as water does upon sugar or salt; it dilutes it, so as to fit it for animal life; nitrogen is so congenial to vegetation as to be considered as the food of plants.

2. What idea can you give of the effect of the weight of the air.

A. It so closely invests the earth with all the bodies therein, that its weight is supposed to be equal to 15 pounds weight upon every square inch.

THE WINDS.

2. What is the wind?

A. Nothing else but the air put violently into motion; and this is occasioned chiefly by means of heat; and by the revolutions of the earth. 2. How does the heat operate?

A. When any part of the air is heated by the sun, or otherwise, it will swell, and thereby affect the adjacent air; and by various degrees of heat in different places, there will arise various motions of the air; the air much heated, will ascend towards the upper part of the atmosphere, and the adjacent air will rush in to supply its place; and therefore there will be a stream or current of air from all parts towards the place where the heat is; hence we see the reason why the air rushes with such force into a glass house, a tile kiln, or into close rooms where great fires are made; and also why smoke is carried up a chimney, and why the air rushes in at the keyhole of a door or small chink, when there is a fire in the room.

2. How are the winds described ?

A. Into four principal ones, the NORTH, SOUTH, EAST and WEST, which receive their names from the points whence they blow.

The winds in various parts of the earth are not of the same temperature, though blowing from the same point of the compass.

In the United States, and in many parts of Europe, the south wind is warm, as it blows from the torrid zone; but at the Cape of Good Hope,

at St. Helena, and in the Indian archipelago, the south wind is cool and refreshing.

In England the east wind is dry because it comes across Asia; with us it is wet because it crosses the Atlantic; while with them the west wind is damp and with us dry from similar

causes.

OF THE MONSOONS.

2. What are the winds called monsoons ? A. They are the winds which blow in the Indian, Pacific, and Atlantic oceans, for nearly one half of the year in one direction, and the remaining part in the opposite direction.

2. Do you know what is the cause of their uniformity and change?

A. I have never heard it accounted for in a satisfactory manner....perhaps it may arise from the rotation of the earth from west to east at the rate of 58,000 miles an hour, which must discompose the atmosphere or air, and carry it in

a current.

2. But then the monsoons should blow either in the same direction with the revolution of the earth, or the contrary....east or west....whereas they blow either south-east or north-west, or nearly so?

A. The obliquity of the ecliptic....the extreme cold at the poles, and the heat at the equator, may combine to give the winds the direction immediately between the cardinal points.

But that does not account for the half year's opposition of the monsoon to the course of the preceding half year?

A. Very true; but the course of the sun in the ecliptic, one half year in the southern and the other in the northern hemisphere, may ex

plain this. Observe....what is here said about the monsoons is merely speculative....to exer cise ingenuity and promote enquiry.

The velocity of the wind is at the rate of 50 or 60 miles an hour in a great storm; that of a common brisk wind is about 15 miles an hour; and some winds move not even one mile in that pace of time.

A person, therefore, on horseback, and even sometimes on foot, may be said to outstrip the wind; for if he moves faster than the wind, which is very possible, he will have a wind in his face, though the motion of the air be really the contrary way.

The air in the form of clouds is often seen to move in two contrary currents, and this happens generally previous to thunder. The clouds in such a case are seen to move one way, while the weather-cock points another.

An experienced seaman has furnished the following account of the ceasing of the North East trade wind, the result of a careful examination of nearly 300 journals; it ceases in January between the 6th and 5th degrees of north latitude: in February between the 5th and 3d; in March and April, between the 5th and 2d; in May between the 6th and 4th; in June in the 10th degree; in August and September between the 14th and 13th degrees, and during the months of October, November and December, they blow as far as the line.

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CLASS IX....LESSON VI.

OF TIDES.

2. Pray what is meant by the tides, or alternate flux and reflux of the sea ?

A. As rivers flow and swell, so also does the sea like them it has its currents, that agitate its waters, and preserve them from putrefaction.This great motion of the earth is called its tides. The waters of the ocean have been observed regularly from all antiquity to swell twice in about four and twenty hours, and as often to subside again.

In its influx the sea generally rises for six hours, when it remains, as it were, suspended, and in equilibrio, for about twelve minutes; at that time it is called high water.

In its reflux the sea falls for six hours, when it remains, as it were, in a like manner, suspended, and in equilibrio, for about twelve minutes; at that time it is called low water.

2. What is the cause of these wonderful appearances?

A. According to Newton, they are occasioned by the attraction of the moon; for the waters immediately under the moon will be attracted up in a heap, whilst the waters on the opposite side of the earth, being but feebly attracted, will be very light if they be very light, they also will rise, and all the neighboring waters flowing into that place, they will swell into a heap or mountain of waters, pointing to the opposite parts of the heavens. Thus does the moon, in once going round the earth in twenty-four hours, produce two tides or swells, and consequently as many ebbs.

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