Page:Cyclopaedia, Chambers - Volume 2.djvu/640

 R I V

( 1023 )

R I V

its ^veiling: The Niger ic felf meeting the Mountains of Nubia, is hid under them> and rifes again on the Weftern Side of the Mountains. Thus alio the Tygris is loft in the Mountain Taurus, &c.

Ariflotk, and the Poets, mention feveral fuch Rivers about Arcadia : Alpheus, a River of Arcadia, is particularly famed.' — ■ This, being fwallowed up in the Ground, is fuppoled by the Greek Authors to continue its Progrefs under Earth and Sea, into Sici- ly; where breaking up near Syracufe, it forms the River Arethufi. The great Reafon of this Opinion is, that every fifth Summer the River Arethufa in Sicily, cafts up the Dung of Cattel about the time of the Celebration of the Olympic Games, in Achaia, when the Dung of Victims was ufed to be call into the Al- pheus.

Some Rivers empty themfelves into the Sea by one Mouth, fome by feveral, — Thus the Danube opens into the Euxine Sea by fe- ven Mouths, the Nile by feven, and the Volga by at leaft feven- ty. The Caufe of this variety of Months Varenius attributes principally to Banks of Sand, &c. form'd therein; which gradu- ally increafing, form Iflands, whereby the Channel is divided in- to feveral Branches.— Indeed, the Antients tell us that rhe Nile formerly only emptf d it felf at one Mouth, call'd the OJlam Ca- nobicum; and add, that the other fix are Artificial.

The Channels of Rivers, except inch as were form'd at the Creation, Varenius endeavours to prove to be all Artificial, and dug by Men.— His Reaibns are, that when a new Spring breaks forth, the Water does not make it felf a Channel, but fpreads over the adjacent Land ; fo that the People lave been neceffitated to cut it a Channel to iecure their Grounds ; and that a great Num- ber of Channels of Rivers are certainly known, from Hiftory, to have been dug by Men, e£-r.

As to the Queftion, whether thofe Rivers which run into o- thers, have made themfelves that way by their own Motion, or have been turn'd thither in Canals cut by Men ? He takes the latter to be the more probable,- and concludes the fame of the Arms or Branches of Rivers ; and of the Turns whereby Iflands are form'd in the Tanais, Volga, &c.

To the Queftion, why we have no Salt-Rivers, when as there are many Salt Springs ? He anfwers, thar*tis becaufe Men having no Occaiion for Salt Water, have not dug Channels to conduct the Water of Salt Springs; Salt being procurable at Ids expence. See Salt.

The Water of moft Rivers flows impregnated with Particles of Metals, Minerals, Sands, oily and fat Bodies, &c— Thus lome Rivers bring Sands intermix'd with Grains of Gold ; of which Kind is, i". a River in Japan; a fecond in the Iflands Leyueo, near Japan; 3 . a Rivulet in Africa, call'd Arroe, breaking out of the Roots of the Mountains of the Moon, wherein are Gold Mines: 4 . a River in Guinea, where the Negroes ieparate the Gold Dull from the Sand, and fell it to the Europeans, who Traffick hither for that very purpofe. 5 . In fome Rivulets neat the City of Mexico, arc Grains of Gold taken up : efpecially af- ter Rain; which is to be underfcood of all the other Rivers; none of which yield any thing coniiderable, except in rainy Sea- ions. 6°. laPeru, Sumatra, Cuba, Hifpauiola, and Guia?ta. Laft- ly, there are feveral Brooks in the Countries about the Alps, ef- pccially Thai, out of whofc Waters Gold is drawn, though there be no Grains confpicuou, therein. Add to this, that the Rhine in many Places brings a Gold, n Mud. See Gold.

As to Rivers that bring Grains of Silver, Iron, Copper, Lead, &c. we find no mention of them in Authors ; though doubtlds there are great Numbers of each ; and many of the medicinal Effects of mineral Waters are doubtlefs owing thereto.

We mult not here omit a River in Germany, which is ordina- rily fuppoled to change Iron into Copper. — The Truth is, there is no real Convcrliun of the Metal ; all that is done is, that the Copper, and Vitriolic Particles in the Water, corrode the Iron, and detaching Parts thereof, by means of the Motion of the Water fucceed in their room.

From this variety in the Mixture of the River Water, refult various Qualities, different fpecific Gravities, different Colours, &c. Sec Minerals ater..

Some Rivers, at certain Scafons of the Year, fwell, ib as to overflow their Banks, and drown the neighbouring Lands. — Of thefe the moft eminent is the Nile, which riles fo as to cover all Egypt, except the Hills. The Inundation begins about the 17th Day of June, and increafes for the fpace of forty Days ; and de- creafes for as many : During which Period the Cities of Egypt, Which are all built on Hills, appear as fo many Iflands.

To thefe Inundations Egypt owes all its Fertility ; the Heavens there affording no Rain, or at leaft none in any refpedt coniide- rable.— Hence as the Inundation is great or imall, Egypt, for that Year is fruitful or barren.

The Antient Greeks, &c. were miftaken as to the Caufe of this Inundation ; no body in thofe Days having travelled up to its Source : But the modern Evgliflj and Portugueze Traders into Con- go, Angola, Monomotapa, &c. have let us into the Secret.— From them we learn that the Spring or Source of the Nile is in a large Lake call'd Zaire, round which are a great Number of huge Mountains, call'd the Mountains of the Moon. Now as thefe lie in the Southern Hernifphcre, their Winter will be at the time

of our Summer : But by reafon of their nearnels to the Equator,' (being ionly io° diftant from it) they never feel any notable Cold : Hence it is, that inftead of Snow in the Winter, they have Rain every Day, at leaft two Hours before, and two after Noon. In effect, the Tops of thefe Mountains are always co- ver'd with Clouds, and the Rains almoft continual. Hence Torrents are conftantly gufhing down from the Mountains ; al! ending in the Lake of Zaire ; whence they flow into the Chan- nel of the Nile, and other Rivers arifing from the fame Lake, as the Cttama, Zaire, &c. Hence the Inundation of the Nile.

The other Rivers, which have any notable Stated Inundations, are ; the Niger, which overflows at the fame Time with the Nile. Leo Africamis fays, it begins on the 15 th Day of June, increafes for 40 Days, and decreafes as long.— The Zaire, a Ri- ver of Congo, proceeding from the fame Lake with theNile, and therefore affected in the fame Manner : The Rio de la Plata of Bra/ll, which Maffeus obferves, overflows at the fame time with the Nile : 1 he Ganges : The Indus ; both which lift overflow in June,Jufy,md Augujt ; at which Times the Natives fave great Quantities of the Water in Ponds, to l'erve them the reft of the Year: Several Rivers flowing out of the Lake Chiamay, into the Bay of Bengal, which overflow in September, Ollober, and Novem- ber. Thefe all bring a very great Fertility with them to the Ground : The River Macoa in Can.beia : The River Parana, or Paranaguaja, which fome will have to be the fame with the Sil- ver River: Several Rivers in Coromandelia, a Part of India, over- flowing in the rainy Months from the great Quantity of Water iffuing off the Mountain Gatis: The Euphrates, which o- verflows Mefopotamia, certain Days in the Year. Laftiy, the Ri- ver Sus in Numidia.

The Rivers moft celebrated for their Length, Breadth, Swift- nels of Current, &c. are,— The Nik, which runs almoft in a ftraight Courfe 2520 Geographical Miles. The Niger, which runs 2400 Miles. The Ganges, 1200 Miles. The Ob, 1600 Miles. The Jenifcea in Af a, about the fame Length with the Ob. The River Orel/ana in America,' 60 Miles broad at its Mouth, and 5000 Miles long. The Rio de la Plata, 80 Miles broad at the Mouth. The Omarranan, another River of Brafil: And the great River of Sr. Laurmce, near 2500 Miles long.

River, in Pbyficks, a Water running by its own Gravity, in a Channel open above.— Such is AE Tab. Hydrojlaticks, Pig. 34-

Lavjs of the Motion of Rivers.

The modern Philofophers endeavour to bring the Motion and Flux of Rivers to prccife Laws; and with this View have ap- plied Geometry and Mechanicks thereto : So that the Doctrine of Rivers is become a Part of the new Philofophy.

'The Italian Authors have diftinguifhed themielves herein, and 'tis chiefly to them we are indebted for the Improvement; par- ticularly S. Gugliehnini, who in his Treatife, Delia Natura de' Pi- mi, has abundance of new Observations and Difcoveries rela- ting thereto.

Rivers, he obferves, ufually have their Sources in Mountains or Elevations of Ground; and 'lis in their Delcent from thefe, that they acquire the Velocity or Acceleration which maintains their fu- ture Current.— In Proportion as they advance further, this Veloci- ty diminifhes ; by reaion of the continual Friction of the Wa- ter againft the Bottom and Sides of the Channel, of the various Obftacles they meet withal in their Progrefs ; and of their arri- ving ar length in Plains, where the Delcent is lefs, and their In- clination to the HorizonjOfconfequer.ee, greater. — Thus the Re- no, a River of Italy, which gave Occaiion in fome Mcafure to thefe Speculations, is found near its Mouth, to have fcarce a Defcent of 52 Seconds.

If the acquired Velocity be quite fpent through the many Ob- ftacles; fo that the Current becomes horizontal; there wiil then nothing remain to propagate the Motion, and continue the Stream, but the Height, or ihe perpendicular PreiTureof the Water which is always Proportional to the Hcighth.— And, happily for us, this Refource increafes as the Occaiion for it increafes: For in Pro- portion as the Water lofes of the Velocity acquired, by the De- fcent, it rifes and augments in Depth.

The upper Parts of the Water of a River, and thofe at a Di- stance from the Banks, may continue to flow from the tingle Caufe or Principle of Declivity, howflnallfocveritbe; for not being de- tain'dbyanyObftaclc, the minuteft Difference of Level will have its Effect : But the lower Parts, which roll along the Bottom, will fcarce be fenlible of fo fmall a Declivity, and will only have what Motion they receive fiom the Preflion of the fuperincum- bent Waters.

The natural Vifcidity and Cohefion of the Particles of Water, and that Implication, as it were, which they feein to have with one another, makes the lower, which are moved by means of the Depth, carry along with them the upper, which in a horizontal Channel wou'd have no Motion at ail, or in a Channel very little inclined, next to none. So that the lower, in this Cafe, com- municate to the upper, a Part of the Motion they have receiv'd from it. Hence it frequently happens that the greateft Velo- city of a River is about the middle of its Depth ; fuch middle 12 D Part,-