TIDES. 



TioK, Whiifun. Sec Whitsun-TV,*-. 

 TIDES, two periodical motions of the waters of the fea ; 

 called alfo thf/ux and reflux, or the ebb and/ow. 



When tlie motion of the water is againll tlic wind, it i3 

 called a ■wind-war J-tide : when wind and tide go in the fame 

 diredion, lenuarJ-iiJ.' : when it runs very ftiong, it is called 

 a liJe-gttte. 



T» tuU a over or up into anyplace, is to go in witli the tide, 

 rither ebb or flood, as long as that lafts ; then to Hay at 

 anchor all the time of contrary tide ; and thus to fct in 

 again with the return of the next tide. 



It is faid loflcw-tide and half-tide, allowing fix hours to 

 a tide, when the tide runs throe hours in the offing longer 

 than it does by the (liore ; but, by longer, tliey do not 

 mean its running more hours ; but that, if it be high 

 water a-ftiore at twelve, it will not be fo in the offing till 

 three. An hour and a half longer make tide and quarter- 

 tide, three-fourths of an hour longer make tide and half- 

 quarter tide, &c. 



When the moon is in the firft and third quarter, i. e. 

 when (he is new and full, the tides are high and fwift, and 

 are called fpring-iidet : when (he is in the fecond and lall 

 quarter, the tides are lower and flower, and called neap- 

 tides. 



Tides, Phenomena of the. The fea is obferved to flow, 

 for certain hours, from fouth towards north ; in which mo- 

 tion, or flux, which lafts about fix hours, the fea gradually 

 fwells : fo that, entering the mouths of rivers, it drives 

 back the river -waters toward their heads, or fprings. 



After a continual flux of fix hours, the fea feems to reft 

 for about a quarter of an hour ; after which it begins to 

 ebb or retire back again from north to fouth, for fix hours 

 more ; in which time, tlie water finking, the rivers refume 

 their natural courfe. Then, after a teeming paufe of a 

 quarter of an hour, the fea again begins to flow, as before ; 

 and thus alternately. 



Thus does the fea ebb twice a day, and flow as often ; 

 but not in the fame hours. The period of a flux and re- 

 flux is 12 hours 50^ minutes ; fo that the tides return 

 later and later each day by 505 minutes, which is the ex- 

 cefs of a lunar day above a folar one, fince 285 lunar days 

 are nearly equal 295 folar ones. So that the fea flows as 

 often as the moon palTes the meridian, both the arc above, 

 and that below the horizon ; and ebbs as often as flie pafTes 

 the horizon, both the eaftern and weftern point of it. 



This farther agreement we obferve between the moon 

 and the fea, that the tides, though conftant, are not equal ; 

 but are greateft, when the moon is in conjunftion or op- 

 pofition to the fun, or at the time of new and full moon ; 

 and leaft, when in quadrature to it. This increafe and di- 

 minution conftitute the fpring and neap tides : the augmenta- 

 tion becomes alfo ftill more obfervable when the moon is in 

 its perigee, or neareft the earth. The loweft as well as the 

 higheft water is at the time of the fpring-tides : the neap- 

 tides neither rife fo high nor fall fo low. 



Laftly, thofe tides are the greateft, which happen in the 

 new and full moon, and the time of the equinoxes, while 

 the moon is in its perigee. Thefe tides are often ftill more 

 increafed by the equinoftial winds, which are fometimes fo 

 powerful as to produce a greater tide before or after the 

 equinox than that which happens in the ufual courfe, at the 

 time of the equinox itfelf. 



Add, that the fame things are obferved throughout moft 

 ot the coafts of Europe ; only that the tides are fo much 

 the Icfs, and happen the later, as the coafts are the more 

 northerly. 



Thefe phenomena of the tides arc admirably accounted 



for, from the principle of gravitation. All we require to 

 their folution is, that the earth and moon, and every particle 

 of them, mutually gravitate towards each other : the rea- 

 fonablenefs of which alFumption, fee under the article 

 Gravity. 



Indeed the fagacious Kepler, long ago, conjeftured this to 

 be the caufe of the tides : " If," lays he, " the earth ceafed 

 to attraft its waters towards itfelf, all the water in the 

 ocean would rife and flow into the moon : the fphcre of thc 

 moon's attraftion extends to our earth, and draws up the 

 water." Thus thought Kepler, in his Introd. ad Theor. 

 Mart. This furmife, for it was then no more, is now 

 abundantly verified in the following theory, firft amply de- 

 duced by Dr. Hallcy from the Newtonian principles. 



However, we may obferve with M. de la Lande (Aftro- 

 nomie, vol. iv. Paris, 1781.) that feveral of the ancients, 

 and among others, Pliny, Ptolemy, and Macrobius, were 

 acquainted with the influence of the fun and moon upon 

 the tides. And Pliny fays exprefsly, that the caufe of 

 the ebb and flow is in the fun, which attradls the waters of 

 the ocean ; and adds, that the waters rife in proportion to 

 the proximity of the moon to the earth. 



Tides, Theory of the. — I. It is obvious, that if the earth 

 were entirely fluid, and quiefcent, its particles, by their 

 mutual gravity towards each other, would form themfelves 

 into the figure of an exadt fphere. 



Suppofe, then, that fome power afts on all the particles 

 of this earth with an equal force, and in parallel direftions, 

 the whole mafs will be moved by fuch a power, but its 

 figure will fufFer no alteration by it ; becaufe all the par- 

 ticles, being equally moved by this power in parallel lines, 

 they will ftill keep the fame fituation with refpeft to each 

 other, and ftill form a fphere, whofe centre will have the 

 fame motion as each particle. Upon this fuppofition, if the 

 motion of the earth round the common centre of gravity 

 of the earth and moon were deftroyed, and the earth were 

 left to the influence of its gravitation toward the moon, 

 the earth falling toward the moon would ftill retain its 

 fpherical figure ; all the parts being equally carried on, and 

 retaining therefore the fame fituation with refpeft to each 

 other. But the effedls of the moon's aftion, as well as the 

 aftion itfelf, on different parts of the earth, are unequal, 

 at all places within the angular diftance of 79^° from the 

 line paiTing through the attrafting body, either in the 

 nearer or in the remoter hemifphere : thofe parts, by the 

 general law of gravity, being moft attrafted which are 

 neareft the moon, and thofe being leaft attrafted which are 

 fartheft from the moon ; while the parts that are at a middle 

 diftance, are attraAed by a mean degree of force ; befides, 

 all the parts are not afted on in parallel lines, but in lines 

 dircfted towards the centre of the moon ; and on thefe 

 accounts the fpherical figure of the earth muft fufFer forae 

 change from the moon's aftion. 



Suppofing the earth to fall towards the moon, and ab- 

 ftrafting from the mutual gravitation of its parts towards 

 each other, and alfo from their cohefion ; it wiU eafily ap- 

 pear, that the parts neareft the moon would fall with the 

 fwifteft motion, being moft attradled, and that they would 

 leave the centre or greater bulk of the earth behind them 

 in their fall, while the more remote parts would fall with 

 the floweft motion, being lefs attrafted than the reft, and 

 be left a little behind the bulk of the earth, fo as to be 

 found at a greater diftance from the centre of the earth 

 than at the beginning of the motion. Whence it is mani- 

 feft, that the earth would foon lofe its fpherical figure, Si^.d 

 form itfelf into an oblong elliptic fpheroid, whofe longeft 

 diameter would point at the centre of the moon. 



If 



