תמונות בעמוד
PDF
ePub
[graphic][merged small]

UNIVERSALIST'S MISCELLANY

. FOR JANUARY, 1820..

NATURAL HISTORY

W HEN we view the surface of our globe, we are stricken with the

appearance of those streams of water which we cali rivers, which at once beautify the prospect, fertilize the soil, and subserve the most valuable purposes of human life. The study of nature has been the employment of wise men in every age; yet the attainments of human research have never been fyly satisfactory to the mind. The greatest philosophers have knowi only a little, guessed at more, and lamented their ignorance of most parts of the works of God. “ The sun ariseth, and the sun goeth down, and pants for the place whence he arose., All things are full of labour-man cannot utter it. All rivers run into the sea, yet the sea is not fuil. Unto the place whence the rivers come, thither they return again. The eye is not satisfied with seeing, nor the ear with hearing.” Such were the reflections of the wisest of the ancient Jews. . . .

Whence are rivers produced? whence do they derive those unceasing stores of water, which continually flow in their spacious channels? This question has divided the opinions of mankind almost beyond any other topic in natural history. Almost every philosopher who has thought upon the subject, has given a solution different from others. But in the controversy on this head, we inay rank the contending parties chiefly under two great leaders, M. De La Hire, a famous French writer, on the one hand; and the great Dr. Hally on the other. The first contends that rivers must be supplied from the sea, strained through the pores of the earth; the second has endeavoured to demonstrate that the clouds alone are sufficient for the supply. Both sides have called mathematics to their aid; and, in the opinion of the by-standers, have shewn, that long and laborious calculations can'at any time be made, bry men of science, to obscure both sides of a question.

VOL. IV.

De La Hire, to shew that the clouds, by rain, are insufficient for itit production of rivers, asserts, that rain never penetrates the surface of use earth above sixteen inches. Hence he infers, that it is impossible for it, in many cases, to sink so as to be found at such considerable depths below. as to give rise to rivers. He grants, indeed, that rain water is often seen to mix with rivers, and greatly to swell their currents; but that a much greater part of it evaporates. “ If, says he, the whole earth were covered with water, evaporation alone would be sufficient to carry oil two feet nine inches of it in a year : and yet we know very well, that hardly nineteen inches of rain water fall in that time; so that evaporation would carry offa much greater quantity than is ever known. The small quantity of rain water that falls in a year is therefore but barely sufficient for the purposes of vegetation. Two leaves of a fig-tree have been found, by experiment, to imbibe from the eartlı, in five hours and a half, two ounces of water. This implies the greit quantity of fluid that inust be exhausted in the maintenance of one single plant. Add to this, thar the waters of the river Rungis do, by calculation, rise to fifty inches, and the whole country from whence these waters are supplied, never receives fifty inches in the year, by rain. Besides this, there are many saltsprings, which are known to proceed iinmediately from the sea, and are subject toits flux and reflux. In short, wherever we dig beneath the surface of the earth, except in a few instances, water is to be found; and it is tvis subterraneous water, which is raised into steam, by the internal heat of the earth, that feeds plants. It is tħis water that distils through the interstices of the earth; and there cooling, forms fountains. It is this subterraneous water also that forms the chief supply of rivers, and pours plenty over the whole earth.” See Hist. de l'Acad. 1713. p. 56.

Dr. Hälly, on the contrary, asserts, that the vapours whiclı are exhaled from the sea, and driven by the winds upon land, are more than sufficient to supply, not only plants with moisture, but also to furnish a sufficiency of water to furnish the greatest rivers. He procured an estimate to be made of the quantity of water emptied at the mouth of large rivers; and of the quantity also raised from the sea by 'evaporatior; and it was found, that the latter by far exceeds the former. This calculation was made by Mr. Mariotte. By him it was found, upon receiving such rain as fell in a year, in a proper vessel, fitted for that purpose, that, one year with another, there might fall about twenty inches of water upon the surface of the earth throughout Europe. It was also computed, that the river Seine, from its source to the city of Paris, might cover an extent of ground, that would supply it annually with above seven billions of cubic feet of this water, formed by evaporation. But, upon computing the quantity which passed through the arches of its bridges in a year, it was found to amount to only two hundred and eighty millions of cubic feet, which is not above a sixth part of the former number. Hence, therefore, it appears, that this river may receive a supply brought to it by the evaporated waters of the sea, six times greater than what it gives back to the sea by its current: and therefore, evaporation is more than sufficient for main :;-... .. canoa, riconos -- ..

. In this manner the sea supplies sufficient humidity to the air for furnishing the earth with all necessary moisture. One part of its vapours fall upon its own bosom, before they arrive upon land. Another part is arrcsted by the sides of inountains, and is compelled, by the rising stream of air, to mount upwards towards the summits. Here it is presently precipitated, dripping down by the crannies of the rocks. In some places, entering into the caverns of the mountains, it collects iu those receptacles, which being once filled, it then overflows; and, breaking out by the sides of the hills, forms single springs. Many of these run down by the valleys, and uniting form little rivulets or brooks; many of these, meeting in one common valley, and gaining the plain ground, become a river; and many of these uniting, make such past bodies of water as the Ganges, the Nile, the Danube, and the Rhine.

But there is still a third part of the vapour exhaled from the sea, which falls upon the lower ground, and furnishes plants with their wonted supply. The circulation does not rešt even here; for it is again

exalted into vapour by the action of the sun; and again returned to · that great mass of waters whence it first arose. T'his, according to Dr. Hally, is the most reasonable hypothesis; and much more likely to be true, than that of those who derive all springs from the filtering of the sea waters through certain imaginary tubes or passages within the earth; since it is well known, that the greatest rivers have their most copious fountains the most remote from the sea. See Phil. Trans. vol. ii. p. 128.

The Doctor's opinion, we believe, is the most generally adopted; yet, after all, it is still pressed with great difficulties; and there is still room to look out for a better theory. The perpetuity of many springs, which always yield the same quantity of water, even when there is least vapour or rain, as well as when there is the greatest, is a strong objection. Derham, in his Physico-Theology, mentions, a spring at Upminster, which he could never perceive by his eye to be diminished, even in the greatest droughts, when all the ponds in the country, as well as an adjoining brook, have been dry for several months together. In the rainy seasons also, it was never overflowed, except sometimes, perhaps, for an hour or so, upon the immission of the external rains. He therefore concludes, that if this had its origin from rain or vapour, there would be found an increase or decrease of its water, corresponding to the causes of its production.

· Thus are we tossed from one hypothesis to another. Must we at last be content to settle in conscious ignorance? This, however mortifying, is often the case after the most painful research in philosophical subjects. But perhaps many of our readers will think, that the origin of rivers is best accounted for by an union of the different schemes of De La Hire and Dr. Hally. Happy however for mankind the vapours continue to arise, the rains to descend, the springs to flow, and the rivers to run. Thus the Deity ceases not to bless his creatures with a profusion of goodness, while the wise men of the world mutually confound each others account of his method of distributing his favours.

Some philosophers, though they are at a loss to account for the origin of rivers, think themselves by no means so as it relates to their formation. Varenius says that rivers are artifical He boldly asserts that their channels have been originally formed by the industry of man. His reasons are, that when a new spring breaks forth, the water does not make itself a channel, but it spreads over the adjacent lands. Thus, says he, men are obliged to direct its course; or, otherwise, nature would never have found one. He enumerates many rivers that are certainly known, from history, to have been dug by men. He alleges, that no salt water rivers are found, because men did not want salt water; and as for salt, that was procurable at a less expence than digging a river for it. Although it costs a speculative man but a smali expence of thinking to form such an hypothesis; yet it may, perhaps, be a trial to the reader's patience to detain him longer upon it.

Though philosophy be thas ignorant, as to the production of rivers, yet, the laws of their motion, and the nature of their currents have been very well explained. All rivers have their source either in mountains or elevated lakts; and it is in their descent from these situations, that they acquire that velocity which maintains their future current. At first their course is generally rapid and headions, but it is retarded in its journey, both by the continual friction against its banks by the many obstacles it meets with to diveli its stream, and by the plains becoming more level as it approaches towards the sea. If this acquired velocity be quite spent, and the plain through which the river passes be intirely level, it will, notwithstanding, still continue to run from the perpendicular pressure of the water, which is always in exact proportion to the depth. This perpendicular pressure is nothing more than the weight of the upper waters pressing the lower out of their places, and consequently, driving them forward, as they annot recede against the stream. As this pressure is greatest in the deepest parts of the river, so we generally find the middle of the stream most rapid; both because it has the greatest motion thus communicated by the pressure, and the fewest obstructions from the banks on either side.

Rivers thus set in motion are almost always found to make their own beds. Where they find the ground elevated, they wear its substance away, and deposit the sediment in the next hollow, so as in time to make the bottom of their channels even. On the other hand the water is continually gnawing and eating away the banks on either side; and this with more force as the current happens to strike more directly against them. By these means, it always has a tendency to render them more strait and parallel to its own course, Thus it continues to rectify its banks, and enlarge its bed; and, consequently, to diminish the force of its stream, till an equilibrium is obtained between the force of the water, and the resistance of its banks, upon which both will remain without any further mutation. Happy is it for man that bounds are thus put to the erosion of the earth by water; and that we find all rivers only dig and widen themselves but to a certain degree. In those plains and large vallies where great rivers flow, the bed of the river is usually lower than any part of the valley. But it often happens, that the surface of the water is higher than many of the grounds that are adjacent to the banks of the stream. If, after inundations, we take a view of some rivers, we shall find their banks appear above water, at a time that all the adjacent valley is overflown. This proceeds from the frequent depositions of mud, and such like substances, upon the banks, by the river's frequently overflowing; and thus, by degrees, they become elevated above the plain ; and the water is often seen higher also. Rivers, as every body has seen, are always broadest at the mouth ; and grow narrower towards their source. But what is less known, and probably more deserving curiosity, is, that they run in a more direct channel as they immediately leave their sources, and that their sinuosities and turnings become morenumerous as they proceed. The savages of North America esteem it a certain sign that they are near the sea when they tind the rivers winding, and every now and then changing their direction. And this is even now become an indication to the Europeans theinselves, in their journies through those trackless forests. As these turns of the river increase as it approaches the sea, it is not to be wondered at that they sometimes, divide, and thus disembogue by different channels. The Danube empties itself into the Black sea by seven mouths; the Nile into the mediterranean by seven, and the great river Wolga into the Caspian by seventy

Buffon says, that the current of rivers is to be estimated very different from the manner in which those writers who have given us mathematical theories on this subject, have represented. They found their calculations upon the surface being a perfect piane, from one bank to the other; but this is not the actual state of Nature in this case ; for rivers in general rise in the middle; and this convexity is according to the rapidity of the stream. Any person, to be convinced of this, need only lay his eye as nearly as he can on a level with the streain, and, looking across to the opposite bank, he will perceive the river in the midst to be elevated considerably above what it is at the edge. This rising in some rivers is often found to be three feet high. To account for this it is supposed the water in the midst of the current loses a part of its weight, by the velocity of its motion; while that at the sides, by its slowness, keeps its natural level. It sometimes happens, however, that this appearance is reversed; for when tides are found to flow up with violence against the current, the greatest rapidity is then found at the sides of the river, as the water there least resists the influx from the sea. On these occasions the river presents a concave rather than a convex surface; and, as in the former case, the middle waters rose in a ridge, in this case they sink in a furrow.

TO BE CONTINUED

[blocks in formation]
« הקודםהמשך »