PART III.

AQUEOUS PHENOMENA.

CHAPTER I.

OF RAIN.

160. RAIN is produced by the rapid union of two or more volumes of humid air, differing considerably in temperature; the several portions in union being incapable of holding the same amount of moisture that each can separately retain.

This circumstance is the result of the law, that the capacity of the air for moisture decreases at a faster rate than the temperature.

161. This effect may be thus illustrated: 4000 cubic inches of air, at the temperature of 86° Fah., can contain no more than 31 grains of moisture, and an equal volume, at 32° Fah., only 73th grains. Now, if the two volumes are mingled together, their average temperature will be 59° Fahrenheit, and the weight of moisture they unitedly possess will be 393th grains. But, at this temperature, 31 grains is all the moisture that 8000 cubic inches of air can possibly retain; since the first portion, by its union with the second, diminished its capacity one-half, while that of the latter was only doubled. The excess, therefore, of 7 grains will be condensed, and descend in the form of water.

What does part third treat of?

What subject is discussed in chapter first?
How is rain produced?

Give the illustration.

162. Rain is the result of such combinations on an extensive scale, and the quantity that falls at any particular time or place, depends upon the difference in the several temperatures of the combining volumes, and the amount of moisture which each separately possesses.

163. Winds are the great natural agents by which such combinations are effected, and these occur most readily, when the currents of air are shifting and variable. Constant winds, blowing steadily from the same quarter and possessing an unchanging temperature, can produce no such admixture, and they are consequently attended with dry weather; except in the case where they strike the sloping sides of lofty mountains, carrying the warm air of the sea and the vales far up into the colder regions of the atmosphere.

164. RAIN-GAUGE. The quantity of rain that falls at any station during a given time, is ascertained by means of the rain-gauge, an instrument which is constructed in a variety of ways.

One of the best consists of a cylindrical, copper vessel, furnished with a float; the rain falling into the vessel raises the float, the stem of which is so graduated that an increase in depth, to the extent of one-hundredth of an inch, can easily be measured.

The greatest annual depth occurs at San Luis, Maranham, 2° 30' S. Lat.; and Vera Cruz ranks next in this respect. At the former place, 280 inches have been observed to fall in the course of a year, and at the latter 278 inches.

165. DISTRIBUTION OF RAIN IN LATITUDE. Since the capacity of the air for moisture increases with its temperature, we should naturally infer, that the higher

What circumstances influence the amount of rain which falls at any place?

What great natural agents effect these combinations?
What is said of variable and constant winds?

What is the rain-gauge ?

Where does the greatest yearly depth of rain occur?

What is the law of distribution in respect to latitude?

the mean temperature of any region, the greater would be the amount of rain which descends upon it.

This is true as a general rule, for the annual depth of rain is found to decrease with the increase of latitude, as will be seen from the annexed list of seven localities, where the rain has been measured.

166. EXCEPTIONS. Although this general relation to latitude exists, it is by no means to be supposed, that the same amount of rain descends yearly upon all regions lying within the same parallels; local causes will have their influence, and create, in many cases, extraordinary departures from the common rule. Thus, Bombay and Vera Cruz possess, nearly, the same position in latitude; but while at the former city, the annual depth of rain varies from 61 to 112 inches, that of the latter ranges from 120 to 278 inches.

This is owing to the following circumstances. Vera Cruz is backed by a chain of lofty mountains, rising beyond the limits of perpetual frost, and hither the hot and humid tropical air is constantly driven by the trade winds, as they sweep from the sea. Hence a great and sudden reduction of temperature occurs amid these icy regions, and the air, no longer capable of absorbing its vast stores of moisture, precipitates an immense quantity of rain.

167. At Bergen, in Norway, it has also been found, that in consequence of the moist south-west winds being checked in their course by the mountains, more than 88 inches of rain descend in a year: a quantity greater

Illustrate.

State the exceptions and the cause.

than that which falls at Calcutta during the same period.

168. DAYS OF RAIN. Though the annual amount of rain is greater in the low than in the high latitudes, the rainy days are not so numerous; as appears by the following table, which presents the average yearly number, within the latitudes indicated.

169. From this circumstance we should readily conclude, that the ordinary rains of the tropical climes must be more powerful than those of the temperate regions; an inference which coincides with observation; for it is stated by Mr. Scott, that at Bombay, there once fell, during the first twelve days of the wet season, thirtytwo inches of rain, a quantity equal to the yearly average of England.

170. This fact is also shown by the sudden rise of brooks and rivulets; a remarkable instance of which is related by Mr. Elphinstone, in the account of his mission to Cabul. It occurred between the Indus and Hydaspes, and is thus described. "On one occasion, the rear-guard, with some of the gentlemen of the mission were cut off from the rest by the sudden swelling of brook, which had been a foot deep when they began t cross. It came down with surprising violence, carrying away some loaded camels that were crossing at the time, and rising about ten feet within a minute. Such was its force, that it ran in waves, like the sea, and rose against the bank in a ridge, like the surf on the coast of Coromandel."

171. DISTRIBUTION IN ALTITUDE. The great stores of atmospherical humidity reside in the inferior strata

What is the rule in respect to days of rain?

Where are rains most powerful?

of the air, and, for this reason, less rain descends upon lofty table-lands and mountains, than upon regions situated lower down in the same latitude.

Thus, in India, on the Malabar coast, twelve degrees from the equator, the annual depth of rain is 136 inches; while at Ootacamund, in the Nhilgerries, a region lying a short distance to the east, in the same latitude, but 8500 feet above the ocean, the yearly amount of rain is only 63.88 inches. Likewise, at Sante Fe de Bogota, New Grenada, a city that enjoys an elevation of 8,800 feet, in the fourth degree of north latitude, the annual quantity of rain is nearly the same as that which falls in Germany, which is about twenty-one inches.

Even slight variations in altitude cause perceptible differences in the quantity of rain. At the Paris Observatory, a rain-gauge is placed in the court, and another upon the terrace, eighty-nine feet above. The mean

annual depth of the rain which fell in the court for a space of ten years, was found to be 22.44 inches, and of that which descended upon the terrace during the same period, only 19.68 inches.

172. RAIN UPON COASTS. We have remarked, that the air above the ocean is always saturated, and that its humidity decreases as we penetrate from the sea-shore into the interior of a country. Conformably to this law, other things being equal, more rain descends upon the coasts than upon the central regions of a country; inasmuch as a less reduction of temperature will here produce a precipitation of moisture.

Besides, when the warm, humid air is borne inland by the winds from the sea, its course is marked by descending showers, and its inherent moisture decreases with its progress. Thus, on the west coast of England, 37 inches of rain fall in the course of a year; while in the interior, upon the eastern side, the annual depth is 25 inches. The maritime and inland regions of France

Give the rule in regard to distribution in altitude. Illustrate.
Compare the rain upon coasts and inland regions.

Why is there a difference? Give instances.