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Show 292 TIDAL WAVE CALLED TIIE BORE. .· d far from the mouth of the river, and then borne by a ccaurrnreen t towards the north. Captain Sab m' e,'i.f 1~ 0un d tl1 at t.l1 e s discoloured by the waters of the Amazon, at the dis-sea wa .1 f · h t f not less than three hundred mi es rom 1ts mout , ance o 'd bl 'd' . where they were still running, with comn ~ra e rap1 tty, m a direction inclined to that of the equatonal current of the The deposits derived from this source appear to have ocean. . . d' . f G . formed a large portion of the mar1t1me 1str1ets o ~lana, and are said to extend even to the mouths of the .onnoco, t en d e grees of latitude farther north, where. thath r1ver also · ·t'ng an annual tribute of earthy matter mto t e sea. IS pour c • • Before we conclude our remarks on the actiOn of the hdes, m st not omit to mention the wave ca1led " the Bore," we u h 'd . . which is a sudden and abrupt influx of t e t1 e mto a ~·1ver or narrow strait. Those rivers are most subject. to this w~ve which have the greatest embouchures in proportiOn to th~ s1ze of their channels; because, in that case, a larger proportiOn of tide is forced through a passage comparatively smaller. For this reason, the Bristol Channel is very subject to t~e B?re, where it is of almost daily occurrence, and at sprmg-tides rushes up the estuary with extraord.inary ra~icl~ty. The same phenomena is frequently witnessed m the pnnc1pal branches of the Ganges, and in the Megna. "In the Hoogly, or Calc~tta river," says Rennell, "the Bore comm~nces at ~oogly Po~nt, the place where the river first contracts 1tself, and 1s pe.rcept1ble above Hooo-ly Town ; and so quick is its motion, that 1t hardly employs fo~r hours in travelling from one to the o.ther, th~ugh the distance is near seventy ~iles. At C~lcutta It sometlm:~ occasions an instantaneous rise of five feet , and both here, a h in every other part of its track, the boats, on its appr~ac ' immediately quit the shore, and make for safety. to the I?1d~~: of the river. In the channels, between th~ tsl~nds m d mouth of the Megna, the height of the Bore IS smd to excee twelve feet. and is so terrific in its appearance, and dange~ous in its conseq' uences, that no boat w·i ll venture to pass ~. s, pm~ t. l t , 'These waves may sometimes cause inundatwns, und- Ic e . t. es an dermine cliff's, and still more frequently sweep away Ie . •) Account of Experiments to determine the Figure of the Eo.rth, &c., P· 446. t Renncll, Phil. Trans., 1781. RELATIVE LEVEL OF TilE ATLANTIC AND PACIFIC. 293 land animals from low shores, whereby they may be carried down, and ultimately imbedded in submarine deposits. There is another question, in regard to the effects of tides and currents, not yet fully determined-how far they may ca.use the mean level of the ocean to vary at particular parts of the coast. It has been supposed, that the waters of the Red Sea maintain a constant elevation of between four and five fathoms above the neighbouring waters of the Mediterranean, at all times of the tide; and that there is an equal, if not greater diversity, in the relative levels of the Atlantic and Pacific, on the opposite sides of the isthmus of Panama. But the levellings recently carried across that isthmus by Mr. Lloyd, to ascertain the relative height of the Pacific Ocean at Panama, and of the Atlantic at the mouth of the river Chagres, have shown, that the difference of mean level between those oceans is not considerable. According to the result of this survey, on which great dependence may be placed#.', the mean height of the Pacific is three feet and a half, or 3. 5fl above the Atlantic, if we assume the mean level of the sea to coincide with the mean between the extremes of the elevation and depression of the tides. For between the extremes of elevation and depression of the greatest tides in the Pacific, at Panama, there is a difference of fl7.44 feet; but the mean difference at the usual spring-tides is fll.flfl feet : whereas at Chagres this difference is only 1.16 feet, and is the same at all seasons of the year. 'l'he tides, in short, in the Caribbean Sea are scarcely perceptible, not exceeding those in some parts of the Mediterranean; whereas the rise is very high in the bay of Panama. But astronomers are agreed, that, on mathematical principles, the rise of the tidal wave above the mean level of a particular sea must be greater than the fa1l below it; and although the difference has been hitherto supposed insufficient to cause an appreciable error, it is, nevertheless, worthy of observation, that the error, such as it may be, would tend to reduce the difference now inferred, from the observations of Mr. Lloyd, *.Mr. Lloycl received from General Bolivar a special commission to survey the Isthmus of Panama, with the view of ascertaining the most eligible line of communication between the two seas. He was assisted by Capt. Falmarck, a Sw.edish o.fficer of engineers; anu tho result of their labours will avpear in the Ph1losoplucal Transactions. |