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snow, and a thunder-storm. It is the common herald of the latter, and may be seen rapidly forming during the calm which precedes a discharge of electricity, swelling to a stupendous magnitude, its protuberances, like the domes of an aerial city, shining with a strong silvery or golden light, finely contrasting with the darkness and density of its central regions. Borne by the currents of air, the cirrostratus is often conducted towards the summit of cumulostratus, and appears cutting through its whole extent.
5. Nimbus-Raincloud.— Any of the preceding modifications of cloud may so increase as to veil the sky completely, and put on an appearance of density, from which an experienced observer will augur rain. But they frequently dissolve without any shower, and no rain falls till another modification has been experienced, which commonly occurs in the case of cumulostratus. After exhibiting a great increase of density, and assuming a louring aspect, the blackness of darkness is followed by a lighter shade, evidencing a fresh disposition of the aqueous particles in the cloud, or the formation of nimbus, from which rain falls. This change may frequently be very distinctly observed when the cloud is over a distant spot; and the transition from considerable blackness to a gray obscurity is sure evidence that the shower has commenced, and may be expected to reach the locality of the spectator, should the wind be blowing in his direction, and the nimbus not be previously extinguished. Hence Virgil's reference to the husbandmen anxious to gather in the harvest:
"So while far off at sea the storm-cloud lours,
The nimbus-the least interesting modification of the clouds to the eye-is first in point of attraction when the rainbow appears upon its front. The precipitation of the aqueous vapours to the earth in the form of rain, is caused by contending aerial currents commingling saturated strata of different temperatures, promoting a condensation of the particles beyond what the air is capable of supporting, when the resulting mass gives out a portion of its moisture, which descends by its own gravity in rain, snow, or hail, according to the temperature of those regions of the atmosphere which it has to traverse. This is the last stage of an extensive pilgrimage which the evaporating forces in action constrain the waters of the globe to undertake through localities apart from its surfacea pilgrimage in which there is no halt, and which some portions of the element are perpetually completing, commencing, and pursuing. How different and far apart the sites which are the starting and terminating points of the journey! Exhaled from the surface of a ruffled ocean or tranquil lake, the aqueous particles ascend invisibly into the upper air, where they are called into sensible existence by a change of temperature, and are built up into the variously-formed beautiful and majestic clouds. These are wafted by the atmospheric currents far apart from the scene of the ascension, change their shape and direction at the will of the winds, pass into a state of invisibility, and again emerge from it as warmer or cooler strata are encountered in their aerial flight, till, perhaps, a thousand leagues away from the spot where the liquid element assumed its vaporous form, that combination of circumstances occurs, which reduces it to its original condition, and deposition ensues upon some thirsty prairie or parching field.
The copiousness and energy of rain depend upon the amount of vapour in the atmosphere, and the gradual or rapid manner in which its particles are brought into mutual contact. We have a slow drizzle in the one case, and a violent shower in the other. The drops of rain vary in size, according to Leslie, from the twenty-fifth to the fourth of an inch in diameter. He remarks, that in parting from the clouds, their descent accelerates,
till the resistance opposed by the air becomes equal to their weight, when they continue to fall with an uniform velocity. The velocity bears a certain ratio to the diameter of the drops; those of a thunder shower, which are large, pouring down faster than those of an ordinary rain. The celerity of a small drop, th of an inch in diameter, he estimates at 11 feet per second, upon acquiring its uniform velocity; that of a larger one, 4th of an inch, at 334 feet. A great number of experiments have verified the remarkable circumstance, that a greater quantity of rain falls upon a low site than upon one a little elevated above it. Thus a rain-gauge placed at the bottom of a hill, will collect a larger amount of water in a given time than another placed upon the summit. Dr. Heberden found that the annual depth of rain at the top of Westminster Abbey was 12.099 inches; at a lower altitude, on the top of a neighbouring house, it was 18.139 inches; and on the ground, in the garden of the house, it was 22.608 inches. M. Arago gives a similar result, from observations made during ten years at Paris. On the terrace of the Observatory the annual depth was 50-471 centimetres, or 19-88 inches; while thirty yards below, in the court of the building, it was 56.371 centimetres, or 22-21 inches. Comparing, however, an extensive tract of mountainous country with a low level district, the annual fall of rain in the former greatly exceeds that in the latter, though contrary to the natural presumption suggested by the fact, that the lower regions of the atmosphere are much more saturated with vapour than the upper. At Keswick in Cumberland a mountainous district the average annual depth of rain is 67.5 inches, while on the sea-coast it is not half that amount. On the Great St. Bernard it is 63.13 inches, and at Paris only 21.26. The description of Judea by the sacred writer, contrasting it with the flat lands of Egypt, though not intended to be philosophic, is in harmony with the teaching of science respecting the important part performed by mountains in the general economy of the earth :"For the land whither thou goest in to possess it, is not as the land of Egypt, from whence ye came out; but the land whither ye go to possess it, is a land of hills and valleys, and drinketh water of the rain of heaven." By arresting the course of the clouds, and producing a condensation of aqueous vapour when a warm current of air lights upon their cold summits, the elevations contribute to precipitate the moisture of the atmosphere, often amid a terrible display of electric phenomena a blaze of fiery honours, and the echo of heart-thrilling sounds.
The statement respecting a greater amount of rain being collected by a low than an elevated guage, in the same time, in a given district, is strikingly illustrated by Mr Miller's report on the amount of precipitation in the lake regions of Cumberland and Westmoreland.
Seathwaite, near Derwentwater,. From June 1846 to November 1847, 242 feet, . 193.69 inches.
The amount collected at a low level is the greatest, because the drops enlarge in their descent. They bring with them the low temperature of the upper regions of the atmosphere, and condense on their surfaces the vapour in the inferior and warmer strata.
1. The average annual fall of rain is the greatest within the tropics. It decreases in quantity as we recede from thence towards the poles, because heat, the cause of vapour, diminishes. In the table, the amount is contrasted as observed in tropical and mean or high latitudes:
Mean and High Latitudes.
Though the amount of precipitation is thus far greater within the tropics than in the temperate zone, yet the number of rainy days is less, because the fall, intense while it lasts, is a periodical event. Two seasons divide the year-wet and dry; and during the dry season, entire months frequently pass away without a shower descending, or a cloud being seen. In the temperate zone, also, passing from south to north, the number of rainy days increases, although the intensity of rain diminishes. The average annual number of days on which rain falls at the places mentioned has been given as follows :
Mean and High Latitudes.
Fngland and West France,
But while in the east of Ireland the rainy days amount to 208 in the year, a far less quantity of rain descends than is precipitated at Gibraltar in 68 days.
North of the equator, the periodical tropical rains fall during the northern declination of the sun, and commence south of the equator with its southern declination, except in India, where the wet and dry seasons are regulated by the monsoons. This is a benign arrangement, offering relief from the rays of a vertical sun by the screen of cloud which is coincidently expanded. In some parts of the American continent, and in the West Indies, two wet seasons mark the year; but one is of much shorter duration, and has lighter showers, than the other. Two periods of rain are also mentioned in relation to Judea ; the "first" or autumnal rains, which fall in seed-time, towards the close of October; and the "latter" or spring rains, in April, after the cold season. "I will give you the rain of
your land in his due season, the first rain and the latter rain, that thou mayest gather in thy corn, and thy wine, and thy oil." These two seasonal events were of vast importance to the Jews, though it is a mistake to suppose that rain seldom falls in Palestine except at those eras. It falls copiously then, and also occasionally through the winter months, its entire cessation being in the interval between May and October. Prominence is given to the two rains referred to, on account of their abundance, and especially the time of their occurrence, the success of the agriculturist depending in a great measure upon those plentiful showers. The periodical tropical rains do not fall for any considerable time without an intermission. After a fine morning, the clouds in general gather towards noon; the shower descends with great violence for four or five hours; and towards sunset, the sky clears, and remains cloudless through the night.
There is a considerable diversity in the amount of rain during the wet seasons in tropical countries, at different places, and in different years. In the ten years from 1817 to 1826 inclusive, at Bombay, the average annual quantity was 78.1 inches; but in the course of 1822 there fell 113 inches, while in 1824 the supply did not rise above 34 inches; and hence came famine and pestilence. At Bombay, also, the gauge has received as much as 16 inches of the 78 in the course of twenty-four hours; and while, there, the average annual quantity is as stated above, at Tellicherry, 12° north latitude, it is 116 inches, and in the delta of the Indus not more than 20 inches. There is great discrepancy between the amount at Calcutta and Benares; 72 inches at the former place, and only 46 at the latter. The greatest fall in those districts appears to take place on the eastern boundaries of the Bay of Bengal, where, in 1825, at Arracan, nearly 60 inches were registered in the month of July, and about 43 in August, from which, by a rough estimate, the annual amount is inferred to be not less than 200 inches. A more extraordinary quantity appears to fall in certain sites on the western continent, as in the forests of Guiana, where incessant rains of four or five months are no uncommon occurrence. The most remarkable instance of excessive rain mentioned by Humboldt is upon the authority of Captain Roussin, who states that more than 160 inches have fallen at Cayenne in the single month of February. A parliamentary report on the sickness among the troops at Sierra Leone in 1828, states that during the three months of June, July, and August, there fell 313 inches; and for the whole year, the amount was estimated at 400 inches. But the greatest fall on record is that observed by Mr Yule at Churra, north-east of Calcutta, among the Khasian Mountains. In the single month of August there fell 264 inches, of which 150 inches fell in the space of five consecutive days. Another observer at the same station measured 500 inches in the space of seven months.
2. Both within the tropics, and near their limits, there are extensive tracts of the globe in which rain is either entirely unknown, or it occurs so rarely, and in such small quantities, that a copious shower is quite a phenomenon. These districts consist for the most part of rocky or sandy deserts, where the highly-heated atmosphere does not contain sufficient moisture to admit of precipitation, under any decrease of temperature. In the New World, the rainless regions comprise portions of California and Guatimala, the Mexican table-land, and the coast-line of Peru. Those of the Old World comprehend an immense territory, stretching from Western Africa through the Sahara, a part of Egypt, Arabia, and Persia, into Beloochistan, with another great zone, north of the Himalayas, including the table-land of Thibet, the desert of Gobi, and a portion of Mongolia.
3. In both continents, likewise, districts within and near the zone of the periodical rains are subject to an occasional intermission, and become rainless for considerable intervals, the drought inflicting terrible suffering upon man and beast. Mr Darwin speaks of the South American droughts being somewhat periodical, for upon comparing the dates of several, he found regular intervals of fifteen years between them. The
period included between the years 1827 and 1830 bears the name of the gran seco, or the great drought, in the state of Buenos Ayres; and on account of the light it throws on the cases where vast numbers of animals of all kinds have been found imbedded together, Mr. Darwin's record of it has great interest. "During this time," he remarks, "so little rain fell that the vegetation, even to the thistles, failed; the brooks were dried up, and the whole country assumed the appearance of a dusty high road. This was especially the case in the northern part of the province of Buenos Ayres and the southern part of St. Fé. Very great numbers of birds, wild animals, cattle, and horses perished from the want of food and water. A man told me that the deer used to come into his courtyard to the well, which he had been obliged to dig to supply his own family with water; and that the partridges had hardly strength to fly away when pursued." Captain Owen, in the account of his surveying voyage, relates a similar effect of drought on the elephants, at Benguela, on the west coast of Africa. They invaded the town in a body, to get possession of the wells, not being able to procure any water in the country. A desperate battle ensued between the inhabitants, amounting to nearly three thousand, and the animals. It terminated in the defeat of the latter, but not until they had killed one man, and maimed a great number. Dr. Malcolmson also states, that during a drought in India, the wild animals entered the tents of the troops at Ellore, and that a hare drank out of a vessel held by the adjutant of the regiment.
"The lowest estimation of the loss of cattle in the province of Buenos Ayres alone, was taken at one million head. A proprietor at San Pedro had previously to these years 20,000 cattle; at the end not one remained. San Pedro is situated in the middle of the finest country; and even now abounds again with animals; yet during the latter part of the gran seco, live cattle were brought in vessels for the consumption of the inhabitants. The animals roused from their estancias, and wandering far southward, were mingled together in such multitudes, that a government commission was sent from Buenos Ayres to settle the disputes of the owners. Sir Woodbine Parish informed me of another and very curious source of dispute; the ground being so long dry, such quantities of dust were blown about, that in this open country the landmarks became obliterated, and people could not tell the limits of their estates. I was informed by an eye-witness that the cattle in herds of thousands rushed into the Parana, and being exhausted by hunger, they were unable to crawl up the muddy banks, and thus were drowned. The arm of the river which runs by San Pedro was so full of putrid carcasses, that the master of a vessel told me that the smell rendered it quite impassable. Without doubt several hundred thousand animals thus perished in the river; their bodies when putrid were seen floating down the stream; and many in all probability were deposited in the estuary of the Plata. All the small rivers became highly saline, and this caused the death of vast numbers in particular spots; for when an animal drinks of such water it does not recover. Azara describes the fury of the wild horses on a similar occasion, rushing into the marshes, those which arrived first being overwhelmed and crushed by those which followed. He adds, that more than once he has seen the carcasses of upwards of a thousand wild horses thus destroyed. I noticed that the smaller streams in the Pampas were paved with a breccia of bones, but this probably is the effect of a gradual increase, rather than of the destruction at any one period. Subsequently to the drought of 1827 to 1832, a very rainy season followed, which caused great floods. Hence it is almost certain that some thousands of the skeletons were buried by the deposits of the very next year. What would be the opinion of a geologist, viewing such an enormous collection of bones, of all kinds of animals and of all ages, thus embedded in one thick earthy mass? Would he not attribute it to a flood having swept over the surface of the land, rather than to the common order of things?"