CHAPTER XVIII.

OF RAIN.

"Thou visitest the earth and waterest it; thou greatly enrichest it with the river of God, which is full of water: thou preparest them corn, when thou hast so provided for it." Ps. 1xv. 9.

§ 1. GENERAL OBSERVATIONS.

491. THE winds, sweeping in all directions, carry with them into the places where they go the temperature and moisture of the regions from which they came. A sea breeze will always be moist, and relatively temperate. It is ordinarily the herald of rain. A land breeze is dry and extreme, whether in cold or heat, and generally presages fair weather. From every part of the land and water surface, and at all temperatures, moisture rises in an invisible form, called vapor, which mingles with the air. Evaporation goes on more rapidly when the air is in motion than when it is calm.

492. The tropical atmosphere contains a vast quantity of water as vapor. This is owing to the heat, which, being always very great, increases the capacity of the air for holding moisture. Even under the most serene sky the air is charged with vapor.

493. The greater part of the atmosphere rests on the ocean; the sea, therefore, has the chief influence in modifying climates and supplying the air with moisture. When air contains all the moisture it is capable of receiving, it is said to be saturated. If its temperature be raised, it will hold more; but if it be lowered, its capacity for vapor is diminished, and a part of the vapor will be condensed and deposited, or precipitated.

491. Of the winds as carriers. Sea breeze. Land breeze. Evaporation. Constant. When greatest.-492. State of the tropical atmosphere with regard to vapor. -493. Chief influence in modifying climates, &c. The air, when sat

494. The prevailing winds of the temperate zones blow towards the poles; they are going from warmer to colder climates; consequently, their capacity for moisture decreases with their temperature, and they must let down more water than they can take up again. The prevailing winds of the torrid zone blow towards the equator; they are going from colder to warmer climates. Their capacity for moisture is therefore constantly on the increase, and they must evaporate from this zone more water than they precipitate upon it again.

495. When two currents of air of different temperature, moving rapidly towards each other, come in contact, a heavy fall of rain takes place, and at the same time a quantity of heat and electricity is disengaged, producing thunder and lightning. As the quantity of vapor is most abundant in tropical regions, the rains are heavier, and the drops are larger, than elsewhere.

496. If a wind charged with clouds flows into a warmer and drier atmosphere, its capacity for vapor is thereby increased, and instantly the clouds are dissipated. This is the case with winds blowing from the Mediterranean towards the Sahä'ra.

497. When moist winds come in contact with a high mountain chain, they are forced to ascend the slopes into a higher and colder atmosphere, which condenses the vapors. The rain flows down along the mountain sides, but the wind itself passes over the mountain ridge, cloudless and dry.

498. The mountain chains are the great condensers placed along the continents to receive the moisture of the winds, to serve as reservoirs for the rain waters, and to distribute them afterwards, as they are needed, over the neighboring plains. Every system of mountains becomes the centre of a system of irrigation invaluable to its adjacent lands. They receive more rain in the heights than on their sides, and more falls at their bases than in the neighboring plains. Mountain chains which run N. and S. have a dry and a rainy side; and the prevailing winds of the latitude determine which is the rainy and which is the dry side.

urated. Effects of raising and lowering the temperature. -494. Courses of the winds of the temperate zones. Torrid zones. - 495. Meeting of two currents of air of different temperatures. Why are rains heavier in tropical regions? 496. A wind charged with clouds flowing into a warmer and drier atmosphere. 497. Moist winds in contact with a high mountain chain. 498. Mountain chains the great condensers. Of what is every system of mountains a centre? Mountains running N. and S. - 499. Excess of precipi

499. In some parts of the earth the precipitation is greater than the evaporation; this is shown by the amount of water carried by every river that runs into the sea, which is considered as the excess of precipitation over evaporation. In other parts of the earth, these operations are equal, as in those inland basins, such as that in which the city of Mexico, Lake Titicä'ca, and the Caspian Sea are situated. In other parts still, we find places, as the Desert of Sahä'ra, in which neither evaporation nor precipitation takes place, and in which we find neither plant nor animal.

§ 2. OF CLOUDS.

500. The REGION OF CLOUDS is at a height varying from one to four miles above the earth's surface. The different currents of air in the lower portions of the cloud region run horizontally on each other; and as they generally differ in moisture, temperature, and motion, the colder condense the vapor of the warmer, and make it visible in the form of cloud. At moderate heights clouds consist of water; but at great elevations they are an assemblage of minute crystals of ice. Clouds are most frequently higher within the tropics than in the temperate zones; and in the latter, they are commonly higher in summer than in winter. The denser clouds are usually formed towards noon, when the vapors are raised up by the ascending currents of air. Notwithstanding the varied forms of clouds, they may be classified under a few principal types. There are three primary forms, viz.: the Cirrus, the Cumulus, and the Stratus.

501. The Cirrus is the highest. It sometimes resembles a white brush, at others it consists of horizontal bands of slender silvery filaments. The height of the cirrus is calculated at 19,500 feet, and is made up of minute particles of ice or snow flakes. Among these clouds, which sometimes appear like fleecy cotton, halos and mock suns are formed, which often precede a change of weather, announcing rain in summer, and frost and snow in winter.

502. The Cumulus, or summer cloud, presents itself in the form of a vast heap of vapors resting on a horizontal base;

tation over evaporation, how shown? Equal. Neither. 500. Height of the region of clouds. Formation of clouds. Clouds at moderate heights. At great elevations. Clouds within the tropics. In summer and winter. Denser clouds, when formed? Primary forms of clouds. 501. The cirrus. - 502. The

hence its name, cumulus, a heap, or pile. It is called the summer-day cloud, from its frequent occurrence at that period, resembling a mountain of snow when lighted up by the beams of the sun. It usually begins to form early in the morning, enlarges as the day advances, attains its greatest magnitude in the hottest part of the day, decreases as the sun declines, and breaks up towards sunset.

503. The Stratus, or fall cloud, consists of horizontal bands near the surface of the earth. It belongs to the night, forming at sunset and disappearing at sunrise.

504. Besides these primary varieties, four secondary forms occur. The Cirro-cumulus is a feathery, accumulated cloud, familiarly known as fleecy, consisting of small, rounded patches, arranged in extensive beds, the parts being quite distinct.

505. The Cirro-stratus consists of bands of filaments more compacted than those of the former, lying inclined, or disposed in horizontal stra'ta. It is sometimes seen cutting the disk of the sun or moon with a dark line.

506. The Cumulo-stratus, or twain cloud, is formed of two or more cumuli united together, and resting on a common stratum. This is the most magnificent variety, which often exhibits a copper tinge, indicating a highly electrical condition of the atmosphere, and precedes the thunder storm.

507. The Nimbus, or rain cloud. Any of the preceding modifications may pass over into the actual rainy cloud, first exhibiting a great increase of density, and a bluishblack tone of color, then putting on a lighter shade, or gray obscurity, and becoming fringed at the edges.

508. Among the various offices which the clouds perform, we find them moderating the extremes of heat and cold, and mitigating climates. At one time they spread themselves out, "covering the earth as with a mantle," thereby preventing radiation from its surface, and keeping it warm. At another time, they interpose between the earth and the sun, and screen it from his scorching rays, to protect the tender plants from his too potent heat, the land from drought.

509. When the condensation of vapor takes place at a considerable height in the atmosphere, or in very cold strata of air, the drops descend in the form of hail or snow. If the congelation of moisture takes place slowly, SNow is formed;

cumulus. — 503. The stratus. Secondary forms. -504. The cirro-cumulus.505. The cirro-stratus. - 506. The cumulo-stratus. -507. The nimbus. —508. Remark on the offices of clouds. 509. Of hail. -510. Of fogs. —511. Of

whilst HAIL appears to be produced suddenly by intense cold in the upper regions. Hail, therefore, frequently occurs in summer, and in hot climates where snow is unknown.

510. FOGS and MISTS are formed from the vapors just rising from warm and moist ground, or the surface of water, and coming in contact with air colder than itself. They are, therefore, more frequent in autumn and at the approach of cold weather, than in spring; and as the temperature of the sea is often higher than that of the atmosphere, they are by no means uncommon on the ocean, more particularly in cold climates, and are very frequent in the polar regions. A remarkable stationary fog occurs off the coast of Newfoundland, which owes its origin to the chilly atmosphere of that station, and the comparative warmth of the waters of the adjacent ocean.

511. DEW is formed when air containing an excess of moisture comes in contact with a surface in a certain degree colder than itself. It is found to be deposited on different substances unequally, and in proportion to their powers of radiation, more on vegetables than on dry sand, and very little on bright metallic surfaces. The deposition of dew will be greatest when a clear, cool evening succeeds a sultry day; little or no dew is formed if the sky be veiled in clouds. Frost is merely the ice of dew, as hail is the ice of rain.

512. In the northern hemisphere, the land and water are nearly equally divided, but in the southern hemisphere there is far more water than land. All the great rivers in the world are in the northern hemisphere, excepting the Amazon, which belongs to both, and the Rî'o de lä Plä'ta, which is in the southern; but the chief evaporating surface is in the southern hemisphere. Late in the autumn, throughout the winter, and in early spring, the sun is pouring his rays with the greatest intensity down upon the seas of the southern hemisphere, and, like a powerful engine, is pumping up water for our rivers.

513. The heat which this heavy evaporation absorbs becomes latent, and with the moisture is carried through the upper regions of the atmosphere until it reaches our climates. Here the vapor is formed into clouds, condensed, and precipitated in rains, snow, and hail. The heat which held this water in a state of vapor is set free, and it is that

dew. Frost.-512. Relative quantity of land and water in the northern and southern hemispheres. Hemisphere of great rivers. Hemisphere of evaporating surface. Region and season of intense action of the sun on the ocean. 513. Latent heat. What tempers our winter climate?-514. Evaporation and