infusion of nutgalls. The chalybeate waters are of different kinds; sometimes the iron is combined with sulphuric acid, more frequently it is in union with carbonic acid; this may be just in sufficient quantity to hold the iron in solution, or it may be in excess, in which case, besides chalybeate, the water possesses acid properties, forming what is called an acidulous chalybeate water. Saline waters are those which contain the saline ingredients generally found in mineral waters, but which have not carbonic acid in excess, and are free from sulphuretted hydrogen and iron, or contain them in very trifling quantity. Saline waters may be subdivided into four kinds, viz.:-Alkaline waters, or those which contain alkali in its free state, or combined with carbonic acid, and which render the vegetable blues green. Hard waters, or those which contain carbonate or sulphate of lime. Salt waters, or those in which muriate of soda abounds. Purgative waters, or those which contain principally sulphate of magnesia. ter, which in the air stood at sixty-two degrees, and it ascended immediately to 196°. In another it rose to 204°; and the bulb of the thermometer being applied to a crevice, ascended in less than a minute to the boiling point. Some fish being caught and put into the spring, were boiled fit for eating in 15 minutes. In the island of Iceland there are many hot springs, and several magnificent spouting springs, which are called geysers. NOTE B.-Thermometers. The Mercury expands by heat and contracts by cold with greater uniformity than even spirit of wine: it is therefore the most proper and the most commonly used for thermometers. There are four different thermometers used at present in Europe; these are, Fahrenheit's, Celsius's, Reaumur's, and De Lisle's. Fahrenheit's thermometer is used in Britain. space between the boiling and freezing points is divided into 180°; but the scale begins at the temperature produced by mixing together snow and common salt, which is 32° below the freezing point; of course the freezing point is marked 32°, and the boiling point 212°. The thermometer of Celsius is used in Sweden; it has been used also in France since the first revolu tion, under the name of the thermometre centigrade. In it the space between the freezing and boiling points is divided into 100°. The freezing point is marked 0, the boiling point 100°. The De Lisle's thermometer is used in Russia. space between the boiling and freezing points is divided into 150°; but the gradation begins at the boiling point, and increases towards the freezing point. The boiling point is marked 0, and the freezing point 150°. The temperatures which we can measure by a mercurial thermometer are confined within narrow limits. For mercury freezes at about 39° below zero, and boils at 660°. Hence we cannot employ it to measure greater heats than 660°, nor greater degrees of cold than 39°. Yet many temperatures connected with our most common processes are much higher than 660°. The heat of a common fire, the temperature at which silver, copper, and gold melts, and many other such points, offer familiar examples. -See Dr. Thomson's Outline of the Science of Heat and Electricity,' 1830, 8vo. Hot springs are most frequent in volcanic regions. No satisfactory explanation of the temperature of these springs, and, above all, of their wonderful equability in this respect, for a very long series of || years, has ever been offered. When they are connected with volcanoes, we naturally ascribe the temperature of the spring to the heat of the volcano; but when they occur at a considerable distance from volcanic countries, such an explanation cannot be applied. Thus the hot spring at Bath has continued at a temperature higher than that of the air, The thermometer known by the name of Reaumur, for a period not less than 2,000 years; yet it is so which was in fact constructed by De Luc, was used far from any volcano, that we cannot, without a very in France before the revolution, and is still used in violent and improbable extension of volcanic fires, Italy and Spain. In it the space between the boiling ascribe it to their energy. There are various decom- and freezing points is divided into 80°. The freezing positions of mineral bodies, which generate consid-point is marked 0, the boiling point 80°. erable heat. These decompositions are generally brought about by means of water; or to speak more properly, water is itself the substance which is decomposed, and which generates heat by its decomposition. Thus, for example, there are varieties of pyrites, which are converted into sulphate of iron by the contact of water, and such a change is accompanied by an evolution of heat. Were we to suppose the Bath spring to flow through a bed of such pyrites, its heat might be occasioned by such a decomposition. Such probably is the way in which those springs, that contain sulphurated hydrogen gas, received their impregnation. But we are pretty certain, that such a supposition will not apply to Bath water: first, because it does not contain the notable quantity of sulphate, or iron, which would be necessary upon such a supposition; and, secondly, because instead of sulphurated hydrogen gas, which would infallibly result from such a decomposition of pyrites, there is an evolution of azotic gas. This evolution of azotic gas, however, is a decisive proof that the heat of Bath water is owing to some decomposition or other which takes place within the surface of the earth; though from our imperfect acquaintance with the nature of mineral strata, through which the water flows, we cannot give any satisfactory information about what that decomposition actually is. In the island of St Miguel, one of the Azores, which exhibits very obvious marks of having abounded in volcanoes, there are a considerable number of hot springs of various temperatures; some boiling hot, others cooler, and some so low that they are used as These results he obtained in the following manner : baths, and have acquired great celebrity for the He took a glass tube about two feet long, with a wonderful cures they have performed. Mr. Masson ball at one end, of an inch and a quarter in dia. informs us, that these springs are surrounded with meter; he filled the ball, and part of the tube, with abundance of native sulphur, which, he affirms, is water which had previously been deprived of air as exhaled by them in abundance-a circumstance which much as possible; he then placed it under the rerenders it probable, that the heat of these springs ceiver of an air-pump, and removed from it the presdepends upon the decomposition of pyrites. In the sure of the atmosphere; under this treatment he island of Amsterdam there are several hot springs; observed that the water rose a little way in the in one of them was immersed Fahrenheit's thermome-tube. On the contrary, when he placed the appara NOTE C.-Compressibility of Water. With the barometer at 291, and thermemoter at 50, Canton declares the following to be the results he obtained: Compression of spirit of wine, 66 parts in a million, 40 3 contends that rivers must be supplied from the sea, strained through the pores of the earth; and Dr. Halley, who has endeavoured to demon De la Hire begins his proofs, that rain-water, evaporated from the sea, is insufficient for the production of rivers: by showing that rain never penetrates the surface of the earth above sixteen inches. From thence he infers, that it is impossible for it in many cases, to sink so as to be found at such considerable depths below. Rain tus upon a condensing engine, and by condensing the air in the receiver, increased the pressure upon the water, he observed that the water descended a little way in the tube. In this manner he found that water expanded one part in 21,740 when the pres-strate that the clouds alone are sufficient for the sure of the atmosphere was removed, and submitted supply. Both sides have brought in mathematics to a compression of one part in 10,870 under the to their aid; and have shown that long and weight of a double atmosphere. He also observed laborious calculations can at any time be made that water possessed the remarkable property of being more compressible in winter than in summer; to obscure both sides of a question. contrary to the effect on spirit of wine and oil of olives. Lest it might be supposed that the compressibility thus discovered might be owing to air lodged within the fluids employed, a quantity of water was caused to imbibe more air than it contained in a preceding trial; but its compressibility was not increased. These experiments, although upon the whole so apparently decisive of the questions they were instituted to determine, are yet not to be received without some caution; and in par-water, he grants, is often seen to mix with rivers, ticular, the remark that the addition of a portion of so compressible a fluid as air, did not render water more compressible than before, is rather staggering, and is calculated to throw the veil of doubt over all the rest. It remains therefore, for future investigation to fix the judgment of philosophers on this sub-to carry off two feet nine inches of it in a year: ject; in the meantime, even granting all the compressibility that has been contended for, the quantity of it is too small to be noticed in practice. Persons at sea frequently try an experiment which proves, in a great degree, the incompressibility of water. Having corked a bottle containing only air, and therefore called empty, they tie a rope to it, and sink it to a considerable depth by a sufficient weight; on pulling up the bottle, they generally find it either broken, or the cork forced in: but on sinking to the same or even any greater depth, a bottle filled with water, they find it, when drawn up, to be uninjured, because the water resists compression, and therefore supports the bottle; which support, under the pressure at a great depth, the air cannot supply. CHAP. XIV. OF THE ORIGIN OF RIVERS. and to swell their currents; but a much greater part of it evaporates. "In fact," continues he, "if we suppose the earth everywhere covered with water, evaporation alone would be sufficient and yet we very well know, that scarcely nineteen inches of rain-water fall in that time: so that evaporation would carry off a much greater quantity than is ever known to descend. The small quantity of rain-water that falls 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 earth, in five hours and a half, two ounces of water. This implies the great quantity of fluid that must be exhausted in the maintenance of one single plant. Add to this, that the waters of the river Rungis will, by calculation, rise to fifty inches; and the whole country from whence they are supplied never receives fifty inches in the year by rain. Besides this, there are many salt springs, which are known to proceed immediately from the sea, and are subject to its flux and reflux. In short, wherever we dig beneath the surface of the earth, except in a very few instances, water is to be found; and it is by this subterraneous water that springs and rivers, nay, a great part of vegetation itself, is supported. It is this subterraneous water which is raised into steam, by the internal heat of the earth, that feeds plants. It is this subterraneous water that distils through its interstices; and there, cooling, forms fountains. It is this that, by the addition of rains, is increased into rivers, and pours plenty over the "THE sun ariseth, and the sun goeth down, and 1 Ecclesiastes, chap. i. ver. 5, 7, 8. On the other side of the question,3 it is asserted, that the vapours which 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 the greatest rivers. For this purpose, an estimate has been made of the quantity of water emptied at the mouths of the greatest rivers; and of the quantity also raised from the sea by evaporation; and it has been found, that 2 Hist. de l'Acad. 1713, p. 56. 3 Phil. Trans. vol. ii. p. 128. year, the latter by far exceeds the former. This calculation was made by Mr. Marriotte. By him it was found, upon receiving such rain as fell in a 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 millions of cubic feet of this water, formed by evaporation. But upon computing the quantity which passed through the arches of one of its bridges in a year it was found to amount only to two hundred and eighty millions of cubic feet, which is not above the sixth part of the former number. Hence 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 maintaining the greatest rivers, and supplying the purposes also of vegetation.* theory. The perpetuity of many springs, which always yield the same quantity, when the least rain or vapour is afforded, as well as when the greatest, is a strong objection. Derham mentions a spring at Upminster, which he could never perceive by his eye to be diminished in the greatest droughts, even 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, justly enough concludes, that had this spring 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 the reader, after having been tossed from one hypothesis to another, must at last be content to settle in conscious ignorance. All that has been written upon this subject, affords him rather something to say, than something to think; something rather for others than for himself. Varenius, indeed, although he is at a loss for the In this manner, the sea supplies sufficient hu- origin of rivers, is by no means so as to their formidity to the air, for furnishing the earth with mation. He is pretty positive that all rivers are all necessary moisture. One part of its vapours artificial. He boldly asserts that their channels falls upon its own bosom, before it arrives upon have been originally formed by the industry of land. Another part is arrested by the sides of man. His reasons are, that when a new spring mountains, and is compelled, by the rising stream breaks forth, the water does not make itself a of air, to mount upward towards the summits. new channel, but spreads over the adjacent land. Here it is presently precipitated, dripping down" Thus," says he, "men are obliged to direct its by the crannies of the stone. In some places, course; or, otherwise, Nature would never have entering into the caverns of the mountain, it found one.' He enumerates many rivers that gathers in those receptacles, which being once are certainly known, from history, to have been filled, all the rest overflows; and breaking out dug by men. He alleges, that no salt-water rivers by the sides of the hills, forms single springs. are found, because men did not want salt-water; Many of these run down by the valleys or guts and as for salt, that was procurable at less expense between the ridges of the mountain, and, coming than digging a river for it. However, it costs a to unite, form little rivulets or brooks; many of speculative man but a small expense of thinking these meeting in one common valley, and gaining to form such an hypothesis. It may perhaps enthe plain ground, being grown less rapid, become gross the reader's patience to detain him longer a river; and many of these uniting, make such upon it. vast bodies of water as the Rhine, the Rhone, and the Danube. There is still a third part which falls upon the lower grounds, and furnishes plants with their wonted supply. But the circulation does not rest even here; for it is again exhaled into vapour by the action of the sun; and afterwards returned to that great mass of waters whence it first arose. "This," adds Dr. Halley, "seems 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.' 195 This seems the most general opinion; and yet, after all, it is still pressed with great difficulties; and there is still room to look out for a better 4 See Supplementary Note A, p. 127. 5 Phil. Trans. vol. ii. p. 128. Nevertheless, though philosophy be thus ignorant as to the production of rivers, yet the laws of their motion, and the nature of their currents, have been very well explained. The Italians have particularly distinguished themselves in this respect; and it is chiefly to them that we are indebted for the improvement.7 All rivers have their source either in mountains or elevated lakes; and it is in their descent from these that they acquire that velocity which maintains their future current. At first their course is generally rapid and headlong; but it is retarded in its journey, by the continual friction against its banks, by the many obstacles it meets to divert its stream, and by the plains generally becoming more level as it approaches towards the sea. If this acquired velocity be quite spent, and the plain through which the river passes is en. 6 Derham Physico-Theol. 7 S. Guglielmini della Natura de Fiumi, passim. And it is happy 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 10 and large valleys 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 overflowed. This proceeds from the frequent deposition of mud, and such like substances, upon the banks, by the rivers frequently overflowing; and thus, by degrees, they become elevated above the plain; and the water is often seen higher also. tirely level; it will, notwithstanding, still conti- | both will remain without any further mutation. nue 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 cannot 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.8 Rivers thus set into motion are almost always found to make their own beds. Where they find the bed 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 each 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 straight 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 there becomes an equilibrium between the force of the water, and the resistance of its banks, upon which Rivers, as everybody 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 more numerous as they proceed. It is a certain sign among the savages of North America, that they are near the sea, when they find the rivers winding, and every now and then changing their direction. 8 Many great rivers, in fact, flow with an almost And this is even now become an indication to imperceptible declivity. The river of the Amazons the Europeans themselves, in their journeys has only ten feet and a-half of declivity upon two through those trackless forests. As those sinuohundred leagues of extent of water, which makes sities, therefore, increase as the river approaches of an inch for every 1,000 feet. The Seine, between Valvins and Serves, has only one foot declivity out the sea, it is not to be wondered at that they of 6,600. The Loire has, between Pouilly and Bri- sometimes divide, and thus disembogue by differare, one foot in 7,500; but between Briare and Or-ent channels. The Danube disembogues into the leans only one foot in 13,596. In East Frizeland, in Euxine by seven mouths; the Nile by the same the United Provinces, two small neighbouring rivers number; and the Wolga by seventy. have, the one of an inch, the other of declivity for every 1,000 feet. The Marwede, between Herdinxveld and Dort, falls an inch along 1,125 feet; but between Dort and the sea, only one inch along 9,000 feet. Even the most rapid rivers have less declivity than is commonly imagined. The Rhine between Schauffhausen and Strasburg has a fall of 4 feet in a mile; and of 2 feet between Strasburg and Schenkenschantz. Hence we see the reason why one river may receive another almost as large as itself, without any considerable enlargement of its bed; the augmentation of its body only accelerates its course. Sometimes one river falling into another with great rapidity, and at a very acute angle, will force the former to retrace its course, and return for a short space towards its source. This has happened more than once to the Rhone near Geneva; the impetuous Arva, which descends from the mountains of Savoy, being swollen beyond its usual size, has made the more gentle waters of the Rhone flow back into the lake of Geneva; causing the wheels of the mills to revolve back wards. Some rivers have no stream whatever, and the cause is easily discovered; the land having scarcely any declivity, does not impart a sufficiently strong impulse to their waters, which are constantly retarded, and finally absorbed by the sand. Sometimes these waters are evaporated by the heat of the sun, as is the case with the rivers of Arabia and Africa; but they more commonly flow into pools, marshes, or salt lakes.-ED. The currents 11 of rivers are to be estimated very differently from the manner in which those writers, who have given us mathematical theories on this subject, represent them. They found their calculations upon the surface being a perfect plain from one bank to the other: but this is not the actual state of nature; for rivers in general rise in the middle: and this convexity is greatest in proportion as the rapidity of the stream is greater. · Any person, to be convinced of this, need only lay his eye, as nearly as he can, on a level with the stream, 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 edges. This rising, in some rivers, is often found to be three feet high; and is ever increased in proportion to the rapidity of the stream. In this case, the water in the midst of the current loses a part of its weight, from the velocity of its motion; while that at the sides, for the contrary reason, sinks lower. It some ! times however happens, that this appearance is reversed; for when tides are found to flow up with violence against the natural current of the water, the greatest rapidity is then found at the sides of the river, as the water there least resists the influx from the sea. On those occasions, therefore, 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. The stream of all rivers is more rapid in proportion as its channel is diminished. For instance, it will be much swifter where it is ten yards broad, than where it is twenty; for the force behind still pushing the water forward, when it comes to the narrow part, it must make up by velocity what it wants in room. It often happens that the stream of a river is opposed by one of its jutting banks, by an island in the midst, the arches of a bridge, or some such obstacle. This produces not unfrequently a back current; and the water having passed the arch with great velocity, pushes the water on each side of its direct current. This produces a side | current, tending to the bank; and not unfrequently a whirlpool; in which a large body of waters are circulated in a kind of cavity, sinking down in the middle. The central point of the whirlpool is always lowest, because it has the least motion: the other parts are supported, in some measure, by the violence of theirs, and consequently rise higher as their motion is greater; so that towards the extremity of the whirlpool, must be higher than towards the centre. If the stream of a river be stopped at the surface, and yet be free below; for instance, if it be laid over by a bridge of boats, there will then be a double current; the water at the surface will flow back, while that at the bottom will proceed with increased velocity. It often happens that the current at the bottom is swifter than at the top, when, upon violent land-floods, the weight of waters towards the source presses the waters at the bottom, before it has had time to communicate its motion to the surface. However, in all other cases, the surface of the stream is swifter than the bottom, as it is not retarded by rubbing over the bed of the river. It might be supposed that bridges, dams, and other obstacles in the current of a river, would retard its velocity. But the difference they make is very inconsiderable. The water, by these stoppages, gets an elevation above the object; which, when it has surmounted, it gives a velocity that recompenses the former delay. Islands and turnings also retard the course of the stream but very inconsiderably; any cause which diminishes the quantity of the water, most sensibly diminishes the force and the velocity of the stream. An increase 12 of water in the bed of the river always increases its rapidity; except in cases of 12 Buffon, vol. ii. p. 62. inundation. The instant the river has overflowed its banks, the velocity of its current is always turned that way, and the inundation is perceived to continue for some days; which it would not otherwise do, if, as soon as the cause was discontinued, it acquired its former rapidity. A violent storm, that sets directly up against the course of the stream, will always retard, and sometimes entirely stop its course. I have seen an instance of this, when the bed of a large river was left entirely dry for some hours, and fish were caught among the stones at the bottom. Inundations are generally greater towards the source of rivers than farther down; because the current is generally swifter below than above; and that for the reasons already assigned. A little river 13 may be received into a large one, without augmenting either its width or depth. This, which at first view seems a paradox, is yet very easily accounted for. The little river, in this case, only goes towards increasing the swiftness of the larger, and putting its dormant waters into motion. In this manner the Venetian branch of the Po was pushed on by the Ferrarese branch and that of Panaro, without any enlargement of its breadth or depth from these accessions. A river tending to enter another, either perpendicularly, or in an opposite direction, will be diverted by degrees from that direction; and be obliged to make itself a more favourable entrance downward, and more conspiring with the stream of the former. The union of two rivers into one, makes it flow the swifter; since the same quantity of water, instead of rubbing against four shores, now only rubs against two. And, besides, the current being deeper, becomes, of consequence, more fitted for motion. With respect to the places from whence rivers proceed, it may be taken for a general rule, that the largest 1 and highest mountains supply the greatest and most extensive rivers. It may also be remarked, in whatever direction the ridge of the mountain runs, the river takes an opposite course. If the mountain, for instance, stretches from north to south, the river runs from east to west; and so contrariwise. These are some of the most generally received opinions with regard to the course of rivers; however, they are liable to many exceptions; and nothing but an actual knowledge of each particular river can furnish us with an exact theory of its current. The largest rivers of Europe are, first, the Wolga, which is about six hundred and fifty leagues in length, extending from Reschow to Astrachan. It is remarkable of this river, that it abounds with water during the summer months of May and June; but all the rest of the year is so shallow as scarce to cover its bottom, or allow a passage for loaded vessels that trade up its |