XXXII. The Ocean and its Properties ii. Alternate Advances and Recessions of the Sea iii. Formation of Coral Islands iv. Supposed Isthmus, between Calais and Dover, &c. THE GALLERY OF NATURE AND ART. PART I. NATURE. BOOK II. GEOLOGY. [CONTINUED.] CHAPTER XXXI. SPRINGS, RIVERS, CANALS, LAKES, CATARACTS, AND INUNDATIONS. SECTION I. Origin of Springs and Rivers. THIS subject is still involved in a considerable degree of obscu rity. The theories which have been chiefly invented to account for it are the three following: I. The conveyance of the water of the ocean through subterraneous ducts or canals, to the place where the spring flows out of the earth, and the fountain on the river commences. Water, however, could never in this simple manner, by any power we are ac quainted with, rise to the surface of the loftiest mountains, where it is traced so abundantly. And to this hypothesis has consequently been added that of subterraneous heats or fires, by which, the water being rarefied, has been conceived to ascend through the bowels of the mountains in the form of vapours, and to be afterwards condensed, and once more rendered liquiescent. Yet no sufficient proof has hitherto been offered either of the existence of such subterraneous fires, or of such cavernous structure, in the mountains supposed to be operated upon. II. The capillary hypothesis: or that of those who conceive that the water ascends from the depths of the sea through the sandy or other pores of the earth, in the same manner as it rises in capillary tubes, in sponge or sugar-loaf, so long as the latter remains undissolved. It is, however, sufficient to observe, in confutation of this hypothesis, that though, in consequence of capillary attraction, the water may reach the top or extremity of the hollow sand or minute tube, it will pass no further; it will neither rise above the surface of the mountain, so as to roll down its lateral surface in torrents, nor constitute cisterns or cavities, in such elevated situations. III. The hypothesis of evaporation or the origin of springs, and rivers from melted snow, rain, dew, and condensed aërial vapours. But is the process of evaporation thus contemplated in the aggre gate, equal to so prodigious an effect? and are those countries or places most remarkable for the number and extent of their rivers, when evaporation exists in the greatest abundance? If we may credit Mr. Williams, the evaporation from the surface, of land covered with trees and other vegetables is one-third greater than from the surface of water; but this has not been confirmed by other philosophers. From his experiments it appears that in Bradford in New England the evaporation during 1772 amounted to 42.65 inches. But from the way that his experiments were conducted, the amount was probably too great. From an experiment of Dr. Watson, made June 2, 1779, after a month's drought, it appears that the evaporation, from a square inch of a grass plat, amounted to 1.2 grains in an hour, or 28.8 in twenty-four hours, which is 0.061 of an inch. In another experiment, after there had been no rain for a week, the heat of the earth being 110°, the evaporation was found almost twice as great, or = 0.108 of an inch in the day: the mean of which two experiments is 0.084 inches, amounting for the whole month of June to 2.62 inches. If we suppose this to bear the same proportion to the whole year, that the evaporation in Dr. Dobson's experiments for June do to the annual evaporation, we shall obtain an annual evaporation amounting to about 22 incles; which is much smaller than the average obtained by Mr. Williams. Mr. Dalton and Mr. Hoyle have offered us experiments still more correctly conducted. They took place in the vicinity of Manchester during 1796, and the two succeeding years: and according to these experiments the quantity of vapour raised in that quarter annually is about 25 inches; and if to this we add five inches for the dew, it will make the average evaporation for the year 30 inches. Now if we consider the situation of England, and the greater quantity of vapour usually admitted to be raised from water, it will not surely be considered as too great an allowance if we estimate the mean annual evaporation over the whole surface of the globe at 35 inches. But 35 inches from every square inch on the superficies of the earth make 94.450 cubic miles, equal to the water annually evaporated over the whole globe. This may be a quantity, altogether sufficient for the formation and supply of those immense masses of water which constitute the largest of those rivers which we shall presently notice in their order. But by what means is this prodigious expanse of vapour converted into rain, in which form alone it can generate rivers, if it generate them at all? Rain never begins to fall while the air is transparent: the invisible vapours first pass their maximum, and are changed into vesi. cular vapours; clouds are formed, and these clouds are gradually dissolved in rain. But clouds are not formed in all parts of the horizon at once; the formation begins at one particular spot, while the rest of the air remains clear as before: the first cloud rapidly increases till it overspreads the whole horizon, and the rai: then commences. Now it is remarkable, that though the greatest quan. tity of vapour exists in the lower strata of the atmosphere, clouds never begin to form there, but always at some considerable height, It is remarkable too, that the part of atmosphere at which they form has not arrived at the point of extreme moisture, nor near |