Rivers whose inundations are produced by regular mills have the greater part of their course either within the tropics or at least between 30' N. lat. and 30' S. lat. It is a known fact, that hi those regions heavy rainfalls daily from three to six months in the year. These heavy rains commence when the tun in its progress from one tropic to the other approaches the zenith of a country, and they continue till it has passed a certain distance from it. In the beginning of the wet season, as this part of the year is called in those countries, the rains are tometimes so heavy that in the course of a day the level country is covered with water a foot deep. The rivers of course soon begin to increase iu their volume of water, and after 8011Ie time they rise to the level of the banks, and begin to run over. These inunda tions generally last from two to four months. They are more regular than those which are produced by the melting of the snow, and in general do not exceed a certain height. The rural economy of those countries in which they take place is founded on the knowledge of thin periodical event, and on the certainty that the inundations will fertilise the Beide by depositing on them a fine mud, which enriches the soil more than the best mannre artificially collected and applied. The supply of fertilising matter may not be due in an equal ratio to all the head-farm/me or all the tributaries of a river. In the important case of the Nile, according to M. !Anent, an eminent scientific traveller and geographer, it is by the Atbara, its first great tributary, that most of the black-earth and slime is brought down which manures the lands in Egypt ; from which It has received also the designation of Bahr-el-Aswad, or the Black River. Whenever the inundations do not rise to the uslial height, which is sometimes the case, a great part of the country which is not covered with water yields little or nothing, and the consequence in dearth and famine. When, on time other hand, the inundations rise higher than usual, they are also injurious to rural economy, by reaching those tracts which are set apart for the cultivation of plants, which cannot bear so much moisture as the districts that are regularly Oaxaca. Titus, in lt31, the river 31enani in Siam rose to an extraordinary height: the inundations reached the large orchards vs Inch f..r many mile* in extent cover the more elevates' tracts along the banks, and afford subsistence to a numerous population. Several lords of fruit-trees were almost destroyed, and for some years the lagers:sterns and dnriana were scarce.
The Indus [II mxncsrax, in (Ines Drv.1 belongs to both of these Wee* of river*, or rather to all three. The elevation of its watery is owing to the periodical melting of the snow and the subsequent rains, and it is subject also to enornmon. occasional Inundations. In the year according to Major Cunningham, in his work on 'Adak, the nets, of water accumulated, and which caused the inundation of the Undo bordering the lower part of its course, was estimated at 20.000,000,000 cubic feet, equal to a volume 100 feet deep, 380 feet wide, and Niko long. A similar inundation, though probably of inferior magnitude, took place in August, 1858, when, at Attock, the water rose 50 feet in seven hours and a half, and attained 90 feet during the day. its affluent, the Cebul river, jtoreed a 'wards for ten hours. \'aluaUe Investigations of the mechanical philosophy of these phenomena, and of the relations between the velocity of a river-stream and that of the tide—or other waves to which it may be subject, by Mr. J. Obbaxd and Archdeacon J. 11. Pratt, will be found in the 'Journal of the Asiatic) Society of Bengal,' N. S., voL xaix. (1SG0, No. id..1. p. 263-282. The latter subject has important relations with that of the ascent of the tide up a river, noticed below.
All the rivers between the tropics which are swollen by periodical rains lie only in one hemisphere, the northern or the southern. In the countries through which they flow the waters are low and the ground dry 'iring part of the year, so as to admit of easy cultivation, and at another season the fielda are fertilised by the inundations. The Nile and the Amazonas are alone exceptions. Though the course of the latter river is in the southern hemisphere, its afiluents extend far to the north and south, into both hemispheres, and probably three fourths of the tropical rains which descend upon South America find their way to that large river. To this circumstance are owing its immense volume of water and its great depth. The Amazonas, proptrly speaking, is never at its lowest level, in the sense in which that term is applied to other rivers. When the northern rivers cease to bring down the supply which is owing to the periodical rains, the southern begin to bring their contributions. This fact seems suffi ciently to explain the immense tracts of alluvial soil which extend along the river to a great distance, but the same circumstance also keeps the moil in a state of continual moisture, and makes it a per petual swamp. Accordingly we find that the banks of that river, which admits of a more extensive navigation than any other river in the world, remain nearly destitute of agricultural settleMents, and are still in the possession of savage tribes. In the northern portion of the
upper course of the Nile, that river appears to be almost stagnant except during the rains, and to consist in the dry season of a series of swamps and lakes, rather than to form a continuous stream.
The rivera which drain the countries between 30" N. latitude and those in which the mean temperature of the winter season does not rise above 30", are subject to occasional inundations. But these over flowing. occur only in those rivers whose upper course lies within mountain-ranges whioh are covered with snow for a considerable part of the year. In such cases, while the snow covens the more elevated portion of the mountain-ranges, a sudden change in the weather, which produces a warn, wind, bring,' great volumes of vapours, which, falling in abundant rain, soon dissolve the snow, and the mountain.strearns pour down their waters with increased volume and velocity. As soon as the eaters reach a level tract, it Ls inundated. As these inundations often take place unexpectedly, they cause great damage. Thus we find that some valleys in the Ozark Mountains, in the United States of North America, are almost uninhabitable, owing to the sudden inundations to which the rivers of that mountain-region are subject. Many rivers however never Inundate the adjacent country, unless a heavy gale of wind should blow dirmtly up the river, and drive the sea into it with great forte. Such inundations are very sudden, and sometimes also extensive, but they are of short duration. rttainrs Rms.] important subject in the history riven', relating to the dis tribution of temperature within certain areas of the earth's surface and the Immediately incumbent atmosphere, and their equalisation in other', and which bears also on the connection of rivers with the welfare of organic nature and the human race, is the variation of their temperature in different part, of their course, both at the same and at different semons. This depends, proximately, on the temperature of their affluents, and may even give information of the physical state of the countries in which they rise, when these are otherwise unknown. Dr. Joseph D. Hooker, hi his ' Himalayan Journals' (vol. ii., p. GO), has given some interesting fats on this point, as presented by the river' which rise in the Sikkim Himalaya. The Teosta, a tributary of Use Megua, or Bralimaputra, is always cool in summer (where its bed Is below 2000 feet in absolute elevation), its temperature being 20° below that of the air ; whereas, in mid-winter, when there is less cloud and the snows arc not melting, it is only a few degrees colder than the air. At Illowensong, of which the elevation is 1500 feet, the mean tenmperature of the Tecate, in that season, was and that of the air 52'.; at that elevation the temperature of tho water rarely exceeds GO' at midsummer. Between the altitudes of 4000 feet and 300 (the plains of India) its mean temperature varies about 10° between January and .Italy; at 6000 feet it varies from 55° to 43° during the same period ; and at 10,000 feet it freezes at the edges in winter, and rises to 50' in duly. In Juno, in descending from 12,000 to 1000 feet, Dr. Hooker found that its temperature did not rise 10°, while that of the air rose 30° or 40°. The temperature of the northern feeders of this river, in some parts of their course, actually rises with the increasing elevation. Thus, the Zemu, during the traveller's stay at its junction with the Thlonok (which has its source in the north-cast snowy flank of Kinehinjunga, one of the three or four highest mountains in the world), was at 46', or 6° warmer than the latter ; at 1100 feet higher it was 4S°, and at 1100 feet higher still it was 49°. "These observa tions," Dr. Hooker &Ida, " were repeated in different weeks, and several times on the same day, both in ascending and descending, and always with the mine result ; they told, as certainly as if I had followed the river to its source, that it rose in a drier and comparatively sunny climate, and flowed among little snowed mountains." Another explorer of nature, Mr. Alfred R. Wallace, has recorded (` Travels on the Amazon and Rio Negro,' p. 431) some facts apparently of the same description, not, however, observed by himself, and which he professes his inability to account for. in the month of May some very cold days are said to occur annually on the Upper Amazon and Rio Negro, sometimes so severe that the inhabitants suffer much, and the fishes in the rivers even die of the cold ; though five or ten degrees of diminution of tem perature, Mr. Wallace remarks, is as much as ever takes place. We conceive the depression of temperature of the local atmosphere affecting the inhabitants to be occasioned by the air brought down in contact with the cooled water of the rivers, the temperature of which must be still lower, and that the cause is the annual melting of the snows about the sources of some of their head-streams.