All large rivers receive many others in the extent of their course; for example, the Danube receives more than 200 rivulets and rivers; but by reckoning only such as are considerable rivers, we shall find that the Danube receives 31, the Wolga 32, the Don 5 or 6, the Nieper 19 or 20, the Duine 11 or 12; so likewise in Asia the Hoanho receives 34 or 35, the Jenisca 60, the Oby as many, the Amour about 40, the Kian, or river Nankin about 30, the Ganges upwards of 20, the Euphrates 10 or 11, &c. In Africa, the river Senegal receives upwards of 20 rivers: the Nile does not receive any rivers for upwards of 500 miles from its mouth; the last which falls therein is the Moraba, and from this place to its source it receives about 12 or 13 rivers. In America, the river Amazons receives more than 60, all of which are very considerable; the river St. Lawrence about 40, by reckoning those which fall into the lakes; the Mississippi more than 40, the Plata more than 50, &c.
There are high countries on the earth, which seem to be points of division marked by nature for the distribution of the waters. In Europe,
the environs of Mount St. Goddard are one of these points; another is situate between the provinces of Belozera and Wologda, in Muscovy, from whence many rivers descend, some of which go to the White Sea, others to the Black, and some to the Caspian. In Asia there are several, in the country of Mogul Tartary, from whence rivers flow into Nova Zembla, others to the Gulph Linchidolin, others to the sea of Corea, others to that of China: and so likewise the Little Thibet, whose waters flow towards the sea of China; the Gulph of Bengal, the Gulph of Cambay, and the Lake Aral; in America, the province of Quito; whose rivers run into the North and South Seas, and the Gulph of Mexico.
In the old continent there are about 430 rivers, which fall directly into the ocean, or into the Mediterranean and Black Seas; but in the new continent not more than 145 rivers are known, which fall directly into the sea: in this number I have comprehended only the great rivers, like the Somme in Picardy.
All these rivers carry to the sea a great quantity of mineral and saline particles, which
they have washed from the different soils through which they have passed. The particles of salt, which are easily dissolved, are conveyed to the sea by the water. Some philosophers, and among the rest Halley, have pretended that the saltness of the sea proceeded only from the salts of the earth, which the rivers transport therein. Others assert, that the saltness of the sea is as ancient as the sea itself, and that this salt was created that the waters might not corrupt; but we may justly suppose that the sea is preserved from corruption by the agitations produced by the winds and tides, as much as by the salt it contains; for when put in a barrel it corrupts in a few days; and Boyle relates, that a mariner, who was becalmed for 13 days, found, at the end of that time, the water so infected, that if the calm had not ceased, the greatest part of his people would have perished. The water of the sea is also mixed with a bituminous oil, which gives it a disagreeable taste, and renders it very unhealthful. The quantity of salt contained in sea water is about a fortieth part, and is nearly equally saline throughout, at top as well as bottom, under the line, and at the Cape of Good Hope; although there are
several places, as off the Mosambique Coast, where it is salter than elsewhere.[317:A] It is also asserted not to be so saline under the Arctic Circle, which may proceed from the amazing quantities of snow, and the great rivers which fall into those seas, and because the heat of the sun produces but little evaporation in hot climates.
Be this as it may, I conceive that the saltness of the sea is not only caused by the banks of salt at the bottom of the sea, and along the coasts, but also by the salts of the earth, which the rivers continually convey therein; and that Halley had some reason to presume that in the beginning of the world the sea had but little or no saltness; that it is become so by degrees, and in proportion as the rivers have brought salts therein; that this saltness is every day increasing, and that consequently, by computing the whole quantity of salt brought by all the rivers, we might attain the knowledge of the age of the world by the degrees of the saltness of the sea.
Divers and pearl fishers assert, according to Boyle, that the deeper they descend into the sea, the colder is the water; and that the cold is so intense at considerable depths, that they cannot remain there so long under water, but
are obliged to come up again much sooner than when they descended to only a moderate one. It appeared to me that the weight of the water might be as much the cause of compelling them to shorten their usual time as the intenseness of the cold, when they descend to a depth of 3 or 400 fathoms; but, in fact, divers scarcely ever descend above an hundred feet. The same author relates, that in a voyage to the East-Indies, beyond the line, at about 35 degrees south latitude, a sounding lead of 30 or 35lb weight was sunk to the depth of 400 fathoms, and that being pulled up again, it had become as cold as ice. It is also a frequent practice with mariners to cool their wine at sea by sinking their bottles to the depth of several fathoms, and they affirm the deeper the bottles are sunk, the cooler is the wine.