« 上一页继续 »
able pressure at the surface of the globe. About 14 pounds upon every square inch is the average pressure, but at every place this pressure is liable to variations arising from movements in the sea of air.
We might at first fancy this would be a heavy load upon us, but we must remember that air is a fluid and presses equally in all directions; therefore it supports rather than depresses us. Water is a fluid, denser and heavier than air; but in water we feel lighter, because we are borne up by it. This is why it usually rains when the pressure of the atmosphere is comparatively low, because it is not enough to hold up the drops of rain. At such times we often feel oppressed, and say the air is heavy, whereas really the atmosphere is lighter than usual, and we want its support.
6. Impure exhalations, which are lighter than atmospheric air, are continually being carried by it away from the surface of the earth, where they would injure or destroy life. A well long shut up, when first reopened, would be fatal to any one who went down it, because it would contain a vapour destructive of life; but after it has been opened a short time, the atmosphere presses in, carries off the noxious vapour, and itself supplies its space.
7. But the most important duty of the atmosphere is that of gathering up and rendering back to the earth the water, which is so necessary to all upon the earth. The air is capable, in its ordinary state, of holding suspended in it a certain quantity of moisture. The warmer the air, the more moisture it can retain. Water, therefore, is constantly ascending in the form of vapour from the sea, from lakes, rivers, and all
moist places on the earth. This is called evaporation. This goes on to such an extent, that the Caspian Sea, which has no outlet to the ocean, does not overflow, although the mighty Volga and other rivers are continually pouring their waters into it. This, too, is the reason why the ocean itself, the receptacle of so many vast streams, still preserves its ancient limits. In this way God has been pleased to keep it within the boundary that it shall not pass; that so the proud waves be stayed. The waters thus collected in the air are borne along by the winds, and collect together in the form of clouds, which float high when the atmospheric pressure is great, and descend in rain when it is diminished.
Since warm air holds more moisture than cold, evaporation takes place more rapidly in hot places. Hence, in warm weather ponds are dried up and rivers fail.
This explains the nature and origin of dew. In a hot summer's day, the moisture from the earth is drawn up into the air; but at night, when the surface of the earth cools, the air round it is chilled, and gives up some of its moisture. The earth being cooler than the air, the vapour forms drops upon it, just as drops are formed in a cool glass of water, if surrounded by the vapour of steam.
The dew is greatest in a clear cool night following a sultry day. Clouds prevent the heat of the air from escaping. It is greatest when the difference between the temperature of the day and night is greatest. This is the case in our climate in spring and autumn. Therefore with us there is most dew at these seasons.
Some substances are colder than others, and on
these more dew falls. The thermometer is generally lower upon grass than upon gravel. Therefore in a garden the grassplots are often wet with dew, while the gravel-walks are dry.
How admirable is this arrangement, by which the refreshing dew waters the herbage which needs it, and passes by the stony soil, to which it would do no good!
The vapour which is thus taken up is deposited in the form of rain or snow on the higher places of the earth. Snow covers the tops of high mountains, and melting as it descends the sides, furnishes water to supply rivers. Water in other parts penetrates crevices in the earth, when it meets with rocky caverns and beds which form natural cisterns and channels, from which it comes forth in springs and wells in various places.
Striking, indeed, are these beautiful provisions made by the Creator of the world, "Who laid the foundations of the earth, that it should not be removed for ever. Thou coveredst it with the deep as with a garment the waters stood above the mountains; they go up by the mountains; they go down by the valleys unto the place which thou hast founded for them. Thou hast set a bound that they may not pass over; that they turn not again to cover the earth. He sendeth the springs into the valleys, which run among the hills; they give drink to every beast of the field the wild asses quench their thirst; by them shall the fowls of the heaven have their habitation, which sing among the branches. He watereth the hills from his chambers: the earth is satisfied with the fruit of Thy works."
TRIUMPHAL arch, that fills't the sky
To teach me what thou art.
Still seem as to my childhood's sight,
Betwixt the earth and heaven.
Can all that optics* teach, unfold
When science from creation's face
And yet, fair bow, no fabling dreams,
When o'er the green, undeluged+ earth
Methinks, thy jubilee to keep,
And the first poet sang.
Optics.] The science which teaches the laws of vision. Undeluged.] Rescued from the deluge.
The world's gray fathers.] Noah and his sons.
Nor ever shall the muse's* eye,
The earth to thee its incense yields,
How glorious is thy girdle cast
As fresh in yon horizon dark,
For faithful to its sacred page,
Nor lets the type grow pale with age,
That first spoke peace to man.
*Muse.] The spirit of poetry.
+ Unraptured.] Without rapture.
Verse 35.] Subject of prophecy in the first ages.
THERE is the utmost variety in the productions of the earth. Plants and animals of different kinds are found in different quarters of the globe, and many which flourish in one part cannot be sustained in