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ON FOUNTAINS AND RIVERS.
Addidit et fontes, immensaque stagna lacusque ;
WHAT a delightful ornament to a country is the winding course of a river! How much more exquisitely enchanting does it render the most beautiful landscape! And of what an unspeakable variety of benefits is it productive to the countries through which it flows! Hence rivers, in all their diversities of scenery, ever appear a favourite theme in poetical composition. Homer seldom mentions the country of any of his great personages, without introducing the principal river that waters it by some distinguishing characteristic. The Eridanus of the ancients (the modern Po) has been celebrated by Virgil, Claudian, and Lucan ; Denham and Pope have immortalized the Thames ; and even the rivers in savage climes, that roll their immensity of waters through vast solitary wilds, have neither been neglected nor unsung by our descriptive poets.
When we contemplate a river at its fountain head, and perceive that, at first, it is nothing more than a little vein of water, oozing from some hill
upon a bed of clay or sand, we naturally inquire into the causes of this phenomenon, or, in other words, into the origin of fountains and rivers. Concerning this subject natural philosophers have formed very different conjectures. Those, who imagine that fountains owe their origin to waters brought from the sea by subterranean ducts, give a tolerable account how they lose their saltness by percolation, as they pass through the earth: but they find great difficulty in explaining by what power the water rises above the level of the sea to the tops of mountains, where springs generally abound; it being contrary to the laws of hydrostatics, that a fluid should rise in a tube above the level of its surface. And Sir Richard Blackmore, in the first book of his poem entitled Creation, while he seems to admit this theory, thus inquires;
Tell by what paths, what subterranean ways,
Des Cartes, in order to solve this difficulty, imagined, that after the water is become fresh by percolation, it is raised out of the caverns of the earth in vapours toward its surface; where meeting with rocks near the tops of mountains, in the form of arches or vaults, it adheres to them, and runs down their sides (like water in an alembic), till it meets with proper receptacles, from which it supples the fountains. Varenius and others imagine, that water may rise through the pores of
the earth, as through capillary tubes, by attraction. They illustrate the rise of sea-water up into mountains and hills, by putting a little heap of sand or ashes, a loaf of bread, or the like, into a basin of water; in which case the sand, &c. will represent the dry land, or an island, and the basin of water the sea around it. Here the water in the basin will rise to or near the top of the heap, in the same manner, and from the same principle, as the waters of the sea, lakes, &c. rise in the hills. The principle of the ascent in both is accordingly supposed to be the same with that of the ascent of liquids in capillary tubes, or between contiguous planes, or in a tube filled with ashes; all which are generally accounted for from the doctrine of attraction. But to each of these theories there are insurmountable objections: and Thomson, whose philosophical descriptions are frequently unexceptionable, has particularly endeavoured to refute the latter.
Some sages say, that where the numerous wave
And, ere they gain its highest peak, desert
Another opinion, which has been very generally received, is that of Mariotte, in his Traité du Mouvement des Eaux, who attributes the rise of springs to the rains and melted snow. According to him, the rain-water which falls upon the hills and mountains, penetrating the surface, meets with clay or rocks contiguous to each other. It runs along these, without being able to penetrate them, till, having arrived at the bottom of the mountain, or at a considerable distance from the top, it breaks out of the ground, and thus forms the springs. But, in order to examine this opinion, M. M. Perrault and de la Hire endeavoured, by an experiment, to estimate the quantity of rain and snow that fell in a year; and the result of their inquiries was that the quantity which fell was not sufficient to supply the rivers; for that those of England, Ireland, and Spain, discharge a greater quantity of water annually, than the rain, according to their experiment, was able to supply. They likewise found, that the quantity of water raised in vapour, one year with another, was thirteen times more than falls in rain; which was a plain indication, that the water of fountains cannot be entirely supplied by rain or melted
Dr. Halley's system is that which seems to have given the most general satisfaction. He attributes the origin of springs to vapours raised by the
action of the sun, as well as by the agitation of the winds, from seas, lakes, &c. He made several experiments in order to show, that vapour is a sufficient fund to supply all our springs, rivers, &c. For instance, in order to find the quantity of water which the Mediterranean receives, he allowed the most considerable rivers that run into it, namely, the Iberus, Rhone, Tyber, Po, Danube, Neister, Boristhenes, Tanais, and Nile, each to furnish ten times as much water as the Thames; not that any of them are in reality so great, but thus to allow for the small rivulets. Now the Thames is found, by calculation, to evacuate 203,000,000 tuns of water daily. All the other nine rivers, therefore, will only evacuate 1827 millions of tuns in a day; which is little more than a third of what he had demonstrated, by his preceding experiments, to be raised in that time in vapour. And hence, this great man has discovered a source, abundantly sufficient for the supply of fountains.
Dr. Halley next proceeds to account for this theory on the principles of evaporation. He considers that if an atom of water was expanded into a bubble, so as to be ten times as big in diameter as when it was in water, that atom would become specifically lighter than the air; and therefore would rise, so long as the warmth, which first separated it from the surface of the water, should continue to distend it to the same degree; and, consequently, that vapours may be raised from the sea in that manner, till they arrive at a certain height in the atmosphere, in which they find air of equal specific gravity with themselves. Here they will float, till, being condensed by cold, they become specifically heavier than the air, and fall down in dew; or, being driven by the winds