Now by the most correct charts, the Po, from its source to its mouth, traverses a tract 380 miles long, and the rivers which fall therein, on each side, proceed from the springs and rivers 60 miles distant from the Po; therefore this great river, and the others it receives, waters a tract 380 miles long, and 120 miles broad, which makes 450,600 square miles, but the surface of all the dry land is 85,490,506 square miles; consequently all the water which the rivers carry to the sea, will be 1974 times greater than the quantity which the Po furnishes; but as 26 rivers equal to the Po furnish a cubical mile of water to the sea in a day, of course 1874 rivers like the Po would supply the sea with 26,308 cubical miles of water in a year, and that in the space of 812 years all the rivers would supply the sea with 21,372,626 cubical miles of water; that is to say, as much as there is in the ocean, and therefore 812 years is only required to fill it.[312:A] The result of this calculation is, that the quantity of water evaporated from the sea, and which the winds convey on the earth, is about 245 lines, or from 20 to 21 inches a year, or about two thirds of a line each day: this is a very trifling evaporation even when trebled, in order to estimate the water which refalls in the sea, and which is not conveyed over the earth. Mr. Halley, in the Phil. Transactions, page 192, evidently shews, that the vapours which rise above the sea, and which the winds convey over all the earth, are sufficient to supply all the rivers in the world. Next to the Nile the river Jordan is the most considerable in the Levant, or even in Barbary; it supplies the Dead Sea with about six million tons of water every day; all this water, and more, is raised by evaporation; for, according to Halley's calculation of 6914 tons evaporated from each mile, the Dead Sea, which is 72 miles in length by 18 broad, must every day lose near nine million tons of water, that is, not only all the water it receives from the river Jordan, but also that of the small rivers which come into it from the mountains of Moab and elsewhere; consequently there is no necessity for its communicating with any other sea by subterraneous canals.[313:A] The most rapid rivers are the Tigris, the Indus, the Danube, the Yrtis, in Siberia, the Malmistra, in Silesia, &c. but, as we have already observed, the proportion of the rapidity of rivers depends upon the declivity and upon the weight and quantity of water; by examining the globe, we shall find that the Danube is much less inclined than the Po, the Rhine, or the Rhone, for the Danube has a much longer course than any of these other rivers, and falls into the Black Sea, which is higher than the Mediterranean, and perhaps more so than the ocean.