TIDES. 



Channel to Boulogne, at the rate of about fifty miles an hour, 

 we may calculate that the mean depth of the channel is 

 aboat twenty-eight fathoms, independently of the magni- 

 tude of the refinances of various kinds to be overcome, 

 which require us to fuppofe the depth from thirty to forty 

 fathoms. In the great river of Amazons, the iffeds of the 

 tides are Hill fenfible at the llreights of Pauxis, 500 miles 

 from the fea, after an interval of feveral days fpent in their 

 pafTage up : for the flower progrefTive motion of the water 

 no more impedes the progrefs of a wave againft the ftrcam, 

 than the velocity of the wind prevents the tranfmiflion of 

 found in a contrary direftion. 



Dr. Young obferves, that fcarcely a fingle inflance oc- 

 curs tliat favours the fuppofition of high water in the open 

 fea being within an hour of the moon's fouthing, as it mull 

 be if the depth were very great ; fo that neither the lieight 

 of the tide, nor the time of high-water, will allow us to 

 fuppofe the fea any where quite 10 deep as four miles. 



The tide that is produced on the weftern coaftsof dirope, 

 in the Atlantic, correfponds to tlie fituation of the moon 

 already defcribed. Thus it is higli-water on the coails of 

 Spain, Portugal, and the weft of Ireland, about the third 

 hour after the moon has paffed the meridian ; from thence it 

 flows into the adjacent channels, as it finds the eafieft paffage. 

 One current from it, e. g. runs up by the fouth of Eng- 

 land, another comes in by the north of Scotland ; they take 

 a coniiderable time to move all this way, and it is high-water 

 fooner in the places to which they firfl come, and it begins 

 to fall at tliofe places, while they are ftill going on to others 

 that are farther in their courfe. As they return, they are 

 not able to raife the tide, becaufe the water runs fafter off 

 than it returns, till, by a new tide propagated from the open 

 ocean, the return of the current is (lopped, and the water 

 begins to rife again. The tide, propagated by the moon in 

 the German ocean, when fhe is three hours pad the meridian, 

 takes about twelve hours to come from thence to London- 

 bridge ; fo that when it is high-water there, a new tide is 

 already come to its height in the ocean ; and, in fome inter- 

 mediate place, it mufl be low water at the fame time. 



Confequently, when the moon has north declination, and 

 we (hould expeft the tide at London to be the greatell 

 when the moon is above the horizon, we find it is leaft ; and 

 the contrary when flie has fouth declination. 



At feveral places it is higii-water three hours before the 

 moon comes to the meridian ; but that tide which the moon 

 puflies, as it were, before her, is only the tide oppofite to 

 that which was raifed by her when (he was nine hours paft 

 the oppofite meridian. 



It would be endlefs to recount all tlie particular folutions 

 which are eafy corollaries from tliis dottrine : as why the 

 lakes and feas, fuch as the Cafpian fea and the Mediterranean 

 fea, the Black fea and Baltic, have either fmall or no very 

 fenfible tides: for lakes are generally fo fmall, that when the 

 moon is vertical fhe attrafts every part of them alike, and 

 therefore no part of the water can be raifed higlier than an- 

 other : and having no communicatioa with the ocean, it can 

 neither increafe nor diminifh their water, in order to rife and 

 fall ; and feas that communicate by fuch narrow inlets, and 

 are of fo immenfe an extent, cannot, in a few hours time, 

 receive and empty water enough to raife or fink their furface 

 any thing fenfibly. 



To demonftrate the excellency of this doArine, the exam- 

 ple of the tides in the port of Bat(ha, in the kingdom of 

 Tonquin, in the Eaft Indies, 20° 50' N. lat. which are fo 

 extraordinary and diflerent from all others we have yet 

 heard of, may fuffice. 



The day in which the moon paflTes the cquinoftial, the 



water ftagnates there without any motion ; as the moon re- 

 moves from the equinodlial, the water begins to rife and fall 

 once a day ; and it is high-water at the fetting of tlie moon, 

 and low-water at her rifiiig. This daily tide increafes for 

 about fevcn or eight days, and then decreafes for as many 

 days by the fame degrees, till this motion ceafes, when the 

 moon lias returned to the cquinottial. When Ihe has 

 paded the equinoftial, and declines toward the fouth 

 pole, the water rifes and falls again as before ; but it is 

 hi^h-water now at the rifing, and low-water at the fetting 

 of^the moon. 



Sir Ifaac Newton, in order to account for this extraordi- 

 nary tide, confiders that there are two inlets to this port of 

 Batlha, one from the Chinefe ocean, betwixt the continent 

 and the Manillas, die other from the Indian ocean, betwixt 

 the continent and Borneo. This leads him to propofe, as a 

 folution of this phenomenon, that a tid^-may arrive at Batfiia, 

 througli one of thcfe inlets, at the tliird hour of the moon, 

 and another through the other inlet fix hours after, at the 

 ninth hour of the moon. For, while ihefc tides are equal, 

 the one flowing in as the other ebbs out, the water mull 

 llagnate ; nov? they are equal when the moon is in theequi- 

 nodtial ; but as foon as the moon begins to decline on the 

 fame fide of the equator with Batflia, it has been (hewn that 

 the diurnal tide muft exceed the nofturnal, fo that two 

 greater and two leffer tides muft arrive at Bavfiia by turns. 

 The difference oi thefc will produce an agitation of the 

 water, wliich will rife to its greateft height at the mean time 

 betwixt the tv»-o greateft tides, and fall lowell at a mean 

 time betwixt the two leaft tides ; fo that it will be high- 

 water about the Cxth houi" at the fetting of tlic moon, and 

 low-water at her rifing. When the moon has got to tlie 

 other fide of the equinoftial, the noAurnal tide will exceed 

 the diurnal ; and, therefore, the high-water will be at the 

 rifing, and low-water at the fetting of the moon. 



The fame principles will ferve to account for other extra- 

 ordinary tides, which, we are told, are obferved in plaoes 

 whofe fituation cxpofes them to fuch irregularities : and, as 

 fome think, for particular currents and winds. See Cur- 

 rent and Winds. 



When the time of high-water at any place is, in general, 

 mentioned, it is to be underftood on the days of the fyzy- 

 gies, or days of new and full moon ; wlien the fun and moon 

 p;ifs the meridian of the place at the fame time. Among 

 pilots, it is cuftomary to reckon the time of flood, or high- 

 water, by the point of the compafs the moon bears on, 

 allowing three quarters of an hour for each point, at that 

 time ; thus, on the full and change days, in places where it 

 is flood at noon, the tide is faid to flov.' N. and S., or 

 at 1 2 o'clock ; in other places, on the fame days, where 

 the moon bears I, 2, 3, 4, or more points to the E. or 

 W. of the meridian, when it is high-water, the tide Is faid 

 to flow on fuch point ; thus, if the moon bears S.E. at flood, 

 it is faid to flow S.E. and N.W. or three hours before the 

 meridian, that is, at 9 o'clock ; if it bears S.W. it flows 

 S.W. and N.E. or at three hours after the meridian ; and 

 in like manner for other points of the moon's bearing. 



The times of high-water in any place fall about the fame 

 hours after a period of about fifteen days, or between one 

 fpring-tide and another ; but during that period, the times of 

 high-water fall each day later by about forty -eight minutes. 



From the obfervations of many perfons there have been 

 collected the times when it is high-water on the days of the 

 new and full moon, on moft of the fea-coafts of Europe, 

 and many other places ; which are ufually put in a table 

 againft the names of the places ; a fpecimen of which is 

 fubjoined. 



A Table 



