RIVER. 



The rivers mod celebrated for their length, breadth, twitt- 

 B efs of current, &c. are, the Nile, which runs almolt m a 

 llraieht courfe two thoufand five hundred and twenty geo- 

 grJLcal miles; the Niger, winch runs two thoufand tour 

 hundred miles; the Ganges, two thoufand miles ; the Bur- 

 rampooter, the fame dittance ; the Ob, f.xteen hundred 

 miles; the .Tenniffee, in Afia, about the fame ength with the 

 Ob ; the river Orellana, in America, fixty miles broad at its 

 mouth, and five thoufand miles long ; the Rio de la Plata, 

 about ninety miles broad at the mouth; the Omarannan, 

 another river of Braf.l i and the great river of St. Laurence, 

 near two thoufand live hundred miles long, bee the account 

 of each river under its refpedhve name. 



Major Rennell, in his " Memoir," has eftimated the pro- 

 portional lengths of courfe of fome of the moft noted rivers 

 M the world by the following numbers : 



European Rivers. 



Thames 

 Rhine 

 Danube 

 Volga 



Afiatic Rivers 



Indus 



Euphrates 



Ganges 



Burrampootcr 



Nou Kien, or Ava 



Jenniifee 



Oby - - • 



Amoor 



Lena 



Hoanho (of China) 



Kian Ken (of ditto) 



(probably) 



Nile 



African River. 



American Rivers. 



5^ 



7 



9^ 



6| 

 8± 

 9* 

 9l 

 9h 

 10 



II 



"? 



«5§ 



12^ 



Miffifippi - 8 . 



Amazons - 5 a 



Bv the itatute of Weltm. 2. cap. 47, the king may 

 crant commiffions for perfons to take care of rivers, and the 

 rtiherv m them ; and the lord mayor of London is to have the 

 confervation in breaches and ground overflown as far as the 

 water ebbs and flows in the river Thames. (4 Hen. V 11. 

 1 ? ) Perfons annoying the river Thames, making 

 (helves there, calling dung in it, or taking away Hakes, 

 boards, timber-work, See. of the banks, incur a forfeiture 

 of d by Mat. 27 Hen. VIII. cap. 18. Commiiftoners were 

 aoDointed to prevent exaftions of the occupiers of locks, 

 Weirs, &c. upon the river Thames, weilward from the city 

 of London, to Cricklade, in the county of Wilts, and for 

 ifcertaining the rates of water-carriages on the faid river, 

 bv ftat 6 & 7 W. HI. and this Itatute is revived, with 

 authority from the commiilioners to make orders and con- 

 ftitutions to be obferved, under penalties, &c. by 3 Geo. II. 



° 4 Bvfl'atutcs 8 Geo. II. cap. 20, and 4 Geo. III. cap. 12, 

 it is made felony, without benefit of clergy, mahcioully to 

 cut down any river or fea-bank, by winch lands may be 

 deftroyed, and to deilroy fluiccs or locks upon navigable 

 rivers. See NusANCE, and LARCENY. 



River, in Phyfcs, denotes a ftream of water running by 

 its own gravity from the more elevated parts ot the earth 



Vol. XXX. 



towards thofe which are more deprefled, in a natural bed 

 or channel open above. 



If this channel is artificial, it is called a canal ; of which 

 there are two kinds, viz. that vvhofe channel is every where 

 open without fluices, tailed an artificial river, and that whofe 

 water is kept up or let off by means of fluices, which is 

 properly a canal. 



Rivers, Hydraulic Theory of. — The theory of moving 

 waters is certainly one of great importance, and has there- 

 fore, for a long time, excited confiderable intereft, as well 

 among praftical engineers, as fpeculativc mathematicians ; 

 yet it muft be acknowledged that it is but very lately any 

 thing approaching to a well-founded theory has been efta- 

 blifhed. One of the firft and molt diftinguifhed of thofe 

 who attempted to reduce the motion and difcharges of 

 rivers to correct principles, was Guglielmini ; and if his 

 theory was falfe and hypothetical, yet he was the means of 

 drawing the attention of philofophcrs to thefe inquiries ; 

 and his dedu&ions, though in many refpects incorrect, 

 are neverthelefs entitled to a place in an article on this 

 fubjeft. 



This author obferves, that rivers have ufually their 

 fources in mountains or elevated lands, and that it is in their 

 defcent from thefe they acquire their velocity, or accele- 

 ration, which maintains their future current. In propor- 

 tion as they advance farther, this velocity diminifhes, on 

 account of the continual friction of the water againft the 

 bottom and fides of the channel, of the various obftacles 

 they meet with in their progrefs, and of their arriving, at 

 length, in plains, where the defcent is lefs ; and their in- 

 clination to the horizon, of confequence, greater. Thus 

 the Reno, a river of Italy, which gave occafion, in fome 

 meafure, to thefe fpeculations, is found, near its mouth, to 

 have fcarcely a defcent of fifty-two feconds. 



If the acquired velocity be quite fpent, through the 

 many obftacles, fo that the current becomes horizontal, 

 nothing will then remain to propagate the motion, and 

 continue the ftream, but the depth, or the perpendicular 

 prefTure of the water, which is always proportional to the 

 depth. And happily for us, this refource increafes, as the 

 occafion for it increafes ; for, in proportion as the water 

 lofes of the velocity acquired by the defcent, it rifes and 

 augments in depth. 



It further appears, fays he, from the laws of motion per- 

 taining to bodies moved on inclined planes, that when 

 water flows freely upon an inclined bed, it acquires « 

 velocity, which is always as the fquare root of the quantity 

 or declivity of the bed. But in an horizontal bed, opened 

 by fluices or otherwife, at one or both ends, the water 

 flows out by its gravity alone ; and the flowing is quickei 

 or flower in a direct ratio of the rcfpe&ive heights of the 

 water, by reafon of the weight of the fuperior waters upon 

 the inferior. Hence it follows, firft, that as much as the 

 declivity of the bed or channel of a river is greater, fo 

 much alfo will the velocity of the flowing waters he pro- 

 portionably increafc d. 



Secondly, as much as the water in an horizontal bed is 

 deeper, fo much will the velocity ot the current be in- 

 creafed ; and this velocity wjll diminifh in proportion to 

 the decreafing depths of the water in the bed. 



Thirdly, abftrafting from the refiftance caufed by the 

 bottom and fides of the bed, as much nearer as the water is 

 to the bottom, fo much will its motion be accelerated ; 

 not only becaufe the inferior waters are more compreffed 

 by the fuperior in proportion to their greater depth ; but 

 alfo becaufe the inferior ones have a greater declivity than 

 the fuperior, by reafon of their greater depth in the bed, 

 S f when* 



