THE EARTH. 



An increase* of water in the bed of the 

 river always increases its rapidity; except in 

 cases of 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 ac- 

 quired 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 lish were caught among the stones 

 at the bottom. 



Inundations are generally greater towards 

 the source of rivers than farther down ; be- 

 cause the current is generally swifter below j 

 than above ; and that for the reasons already 

 assigned. 



A little river b may be received into a large I 

 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 th-3 swiftness of the larger, and put- 

 ting its dormant waters into motion. In this 

 manner the Venetian branch of the Po was 

 pushed on by the Ferrarese branch and that 

 of Penaro, without any enlargement of its 

 breadth or depth from these accessions. 



A river tending to enter another, either j 

 perpendicularly, or in an opposite direction, j 

 will be diverted by degrees from that direc- j 

 tion ; arid 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, i 

 the current being deeper, becomes, of con- | 

 sequence, more fitted for motion. 



With respect to the places from whence 

 rivers proceed, it may be taken for a general 

 rule, that the largest" and highest mountains ' 

 supply the greatest and most extensive rivers. 

 It may also be remarked, in whatever direc- 

 tion the ridge of the mountain runs, the river 



Buffon, vol. ii. p. 62. b Guglielmini. 



takes an opposite course. If the mountain, 

 for instance, stretches from north to south, 

 the river runs from east to west; and so con- 

 trariwise. These are some of the most ge- 

 nerally 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 fur- 

 nish 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 Rcschow to 

 Astrachan. It is remarkable of this river, 

 that it abounds with water during the sum- 

 mer 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 ves- 

 sels that trade up its stream. It was up this 

 river that the English attempted to trade into 

 Persia, in which they were so unhappily dis- 

 appointed, in the year 1741. The next in 

 order is the Danube. The course of this is 

 about four hundred and fifty leagues, from the 

 mountains of Switzerland to the Black Sea. 

 It is so deep between Buda and Belgrade, 

 that the Turks and Christians have fleets of 

 men of war upon it ; which frequently en- 

 gaged during the last war between the Otto- 

 mans and the Austrians : however, it is un- 

 navigable further down, by reason of its cata- 

 racts, which prevent its commerce into the 

 Black Sea. The Don, or Tanais, which is 

 four hundred leagues from the source of that 

 branch of it called the Softna, to its mouth in 

 the Euxine Sea. In one. part of its course, it 

 approaches near the Wolga ; and Peter the 

 Great had actually begun a canal, by which 

 he intended joining those two rivers ; but 

 this he did not live to finish. The Nieper, or 

 Boristhenes, which rises in the middle of Mus- 

 covy, and runs a course of three hundred and 

 fifty leagues, to empty itself into the Black 

 Sea. The Old Cossacks inhabit the banks 

 and islands of this river; and frequently cross 

 the Black Sea, to plunder the maritime places 

 on the coasts of Turkey. The Dwina, which 

 takes its rise in a province of the same name 

 in Russia, that runs a course of three hundred 

 leagues, and disembogues into the White Sea, 

 a little below Archangel. 



c Doctor Halley. 



