TIDES. 



water, where it i> (foreximple).t\vo miles deep, will fufficc 

 to raife its furface ten or twelve feet in s tide's time ; where- 

 as, if the lame (juantity of water were to be conveyed 

 throujjh a channel forty fathoms deep, it would require a very 

 grejt ftream to effert it in fo large inlets as are the channel of 

 England and the German ocean ; wlience the tide is found 

 to let ilrongtfll in thofe places where the fea grows narroweft, 

 the fame Quantity of water being, in that cafe, to pafs 

 through a imaller patfage. 



This is mod evident in the ftreights between Portland 

 and Cape la Hogue in Normandy, w'liere the tide runs like a 

 (luice ; and would be yet more between Dover and Calais, if 

 the tide, coming round the ifland, did not check it. 



Thii force, being once impreffed upon the water, conti- 

 nues to carry it above the level of the ordinary height of the 

 ocean, particularly where the water meets a direft obftacle, 

 as it does in St. Maloes ; and where it enters into along 

 channel, which running far into the land, grows very 

 Hriight at its extremity, as it does into the Severn fea at 

 Chepftow, where the tide rifcs to 40 feet, and Briftol, where 

 its height is 30 feet. At Breft, the height of the tides is 

 about JO feet ; at St. Maloes, 50; at Annapolis, in the bay 

 of Fundy, as much fomctimes as 100 feet. In the Medi- 

 terranean, the tides are generally inconfidcrable ; nevcr- 

 thelefs they are perceptible : at Naples, they fometimes 

 rife to a foot ; at Venice, to more than two feet ; and 

 in the Euripus, for a certain number of days in each luna- 

 tion, they are very dillinftly obfervable from the cur- 

 rents which they occafion. In the Weft Indies, and alfo 

 in the gulf of Mexico, the tides are lefs obfervable than in 

 the neighbouring feas, perhaps on account of fome combina- 

 tions derived from the variations of the depth of the rivers, 

 and from the different channels by which they are propa- 

 gated. 



The (hoalnefs of the fea, and the intercurrent continents, 

 are the reafons that in the open ocean the tides rife but to 

 very fmall heights in proportion to what they do in wide- 

 mouthed rivers, opening in the dircdtion of the ftream of the 

 tide ; and that high-water is not at the time of the moon's 

 appulfe to the meridian, but always fome hours after it, as it 

 is obferved upon all the wcftcrn coafts of Europe and Africa, 

 from Ireland to the Cape of Good Hope ; in all which a 

 fouth-weft moon makes high water ; and the fame is reported 

 to hold in the weft' of America. 



So that tides happen to different places at all diftances of 

 the moon from the meridian, and confequently at all hours of 

 the lunar day. 



It is to be confidered that, in order to allow the tides their 

 full motion, the ocean, in which they are produced, ought 

 to be extended from eaft to weft 90° at leaft. Becaufe the 

 places, where tlie moon rifes moft, and mcjft deprelTes the 

 water, are at that diftance from each other. Hence it ap- 

 pears, that it is only in the great oceans tliat fuch tides can 

 be produced, and why in the larger Pacific ocean they ex- 

 ceed thofe in the Atlantic ocean. Hence alfo it is obvious, 

 why the tides are not fo great in the torrid zone, between 

 Africa and America, where the ocean is narrower, as in the 

 temperate zones on either fide ; and we may hence alfo un- 

 derftand, why the tides are fo fmall in iflands that are very 

 far diftant from the ftiores. It is manifeft, that, in the At- 

 lantic ocean, the water cannot rife on one (hore but by de- 

 fcending on the other ; fo that, at the intermediate diftant 

 iflands, it muft continue at a mean height betwixt its ele- 

 vation on one and on the other (hore. But when tides pafs 

 over (hoals, and through ftreights into bays of the fea, their 

 motion becomes more various, and their height depends on 

 many circumftances. 



The tide entering the Atlantic appears, fays Dr. Young, to 

 advance northwards at the rate of about 500 miles an hour, 

 correfponding to a depth of about three miles, fo as to reach 

 Sierra Leone at the eighth hour after the moon's fouthing ; 

 this part of Africa being not very remote from the meridian 

 of the middle of the South Atlantic ocean, and having little 

 (hare in the primitive tides of that ocean. The foutheni 

 tide feems then to ])afs by Cape Blanco and Cape Bojador, to 

 arrive at Gibraltar at the thirteenth hour, and to unite it3 

 effefts with thofe of other tides at various parts of the 

 coafts of Europe. 



We may therefore confider the Atlantic as a detached fea, 

 about ■5500 miles broad, and three miles deep ; and a fea of 

 thefe dimenfions is fufceptible of tides confiderably larger 

 than thofe of the ocean, but how much larger we cannot 

 determine without more accurate meafures. Thefe tides 

 would happen on the European coafts, if there were no re- 

 fiftance, a little lels than five hours after the moon's fouth- 

 ing, and on the coaft of America, a little more than feven 

 hours after ; but the refiftance oppofed to the motion of the 

 fea may eafily accelerate the time of high-water in both 

 cafes about two hours, fo that it may be a little before the 

 third hour on the wellern coafts of Europe and of Africa, 

 and before the fifth on the moft expoled parts of the eafteni 

 coaft of America ; and in the whole of the Atlantic, this 

 tide may be combined more or lefs both with the general 

 fouthern tide, and with the partial effefts of local elevations 

 or depreffions of the bottom of the fea, which may caufe 

 irregularities of various kinds. The fouthern tide is, how- 

 ever, probably lefs confiderable than has fometimes been fup- 

 pofed, for, in the latitudes in which it muft originate, the 

 extent of the elevation can only be half as great as at the 

 equator ; and the iflands of Kerguelen's land and South 

 Georgia, in the latitudes of about co° and 55°, have their 

 tides delayed till the tenth and eleventh hours, apparently 

 becaufe they received them principally from diftant parts oi 

 the ocean, which are nearer to the equator. 



On the weftern coafts of Europe, from Ireland to Cadiz ; 

 on thofe of Africa, from Cape Coaft to the Cape of Good 

 Hope ; and on the coaft of America, from California to the 

 ftreights of Magellan, as well as in the neighbouring iflands ; 

 it is ufually high-water at fome time between two and four 

 hours after the moon's fouthing ; on the eaftern coaft ef 

 South America, between four and fix ; on that of North 

 America, between feven and eleven ; and on the eaftern coafts 

 of Afia and New Holland, between four and eight. The 

 Society Iflands are perhaps too near the middle of the Pa- 

 cific ocean to partake of the eifefts of its primitive tide, and 

 their tide, being fecondary, is probably for tliis reafon a few 

 hours later. At the Almirantes, near the eaftern coaft of 

 Africa, the tide is at the fixth hour ; but there feem to be 

 fome irregularities in the tides of the neighbouring iflands. 



The progrefs of a tide may be very diftinftly traced from 

 its fource in the ocean into the narrow and {hallow branches 

 of the fea which conftitute our channels. Thus the tide is an 

 hour or two later at the Scilly Iflands than in the Atlantic, 

 at Plymouth three, at Cork, Briftol, and Weymouth four, 

 at Caen and Havre fix, at Dubhn and Brighthelmftone 

 feven, at Boulogne and Liverpool eight, at Dover near nine, 

 at the Nore eleven, and at London-bridge twelve and a half. 

 Another portion appears to proceed round Ireland and Scot- 

 land into the North fea ; it arrives from the Atlantic at Lon, 

 donderry in about three hours, at the Orkneys in fix, at Aber- 

 deen in eleven, at Leith in fourteen, at Loeftoffe in twenty, and 

 at the Nore in about twenty-four, fo as to meet there the fub- 

 fequent tide coming from the fouth. From the time occu- 

 pied by the tide in travelling from the mouth of the Englifli 



Channel 



