Point, anti another along the shore near Ceuta , anti there is aIhird current in the middle different from both. Its general direc-tion is from the Atlantic Ocean into the Mediterranean Sea , but*t sometimes comes out when the ebb-title in the Atlantic is con-std^rable. Suppose the moon over the middle of the Mediter-ranean . The surface of the sea will lie level, and it will be half-title at both ends, and therefore within the Straits of Gibraltar .Put without the Straits it is within half an hour of high water,therefore there will be a current setting in from the Atlantic .About three and a half hours after, it is high water within, andPalf-ebb without. The current now sets out from the Mediter-ranean : three hours later, it is low’ water without the Straits andhalf-ebb within; therefore the current has been setting out all thisPrae: three hours after it is halt llood without the Straits andlow water within, and the current is agaih setting in, &c. Werethe earth fluid to the centre, the only sensible motion of theWaters would be up and down, like the waves on the open ocean,1 *hich are not brushed along by strong gales. But the shallowness°f the channel makes a horizontal motion necessary, that Waterln ay be supplied to form the accumulation of the tide. Whenthis is formed on a flat shelving coast, the water must flow in and°ut, on the flat and sands, while it rises and falls. These hori-*°ntal motions must be greatly modified by the channel or bedalong which they move. When the channel contracts along theline of flowing water, the wave, as it moves up the channel, andj s checked by the narrowing shores, must be reflected back, andkeep a-top of the waters still flowing in underneath. Thus it mayr!s e higher in these narrow seas than in the open ocean. This mayServe to explain a little the great tides which happen on somec °asts, such as the coast of Normandy . At. St. Malo the floodfrequently rises 50 feet. But we cannot give any thing like afrll or satisfactory account of these singularities. In the Bay of*undy, and particularly at Annapolis Royal , the water sometimesr ' s es above 100 feet. This seems quite inexplicable by any forcethe sun and moon, which cannot raise the waters of the free0c ean move than eight feet. These great floods are unquestionably?wing to the proper timing of certain oscillations or currents ad-joining, by which they unite, and form one of great force. Such'[iolent motions of water are often seen on a small scale in the mo-tions of brooks and rivers; but we are too little acquainted withhydraulics to explain them with any precision. It is ascertainedthat there is an oscillation of waters formed under the sun and•noon; and that in consequence of the rotation of the earth'Ton her axis, and because of the inertia and the want ofPerfect fluidity of the waters, and obstructions in the channel,ta 's accumulation never reaches the place where 1 it would finallySet Ue if the earth did not turn round her axis. The consequencethis must be a general current of the waters from E. to W.P's may be seen in another way. T he moon in her orbit roundearth has her gravity to the earth diminished by the sun’s dis-[Jrbing force, and therefore moves in an orbit less incurvated,, an she would describe independent of the sun’s action. ShePerefore employs a longer time. If the moon were so near thej. ar th as almost to touch it, the same thing would happen. There-®. r ® suppose the moon turning round the earth, almost in contactAh the equator, with her natural undisturbed periodic time, anda t tiie earth is revolving round her axis in the same time, thesu°f n VV0U P1 remain continually above the same spot of the earth’spj ace (suppose the city of Quito ), and a spectator in another1 , would see the moon always covering the same spot. Nowi the sun act. This will’not affect the rotation of the earth,o ^ c ause the action on one part is exactly balanced by tire actionr a, j°ther. But it will affect the moon. It will move slowerb e i • tlle earth’s centre, and at a greater distance. It will be leftt|, UlK ' by the city of Quito , which it formerly covered. And asst ® ar th moves round from W. to E. the moon, moving morelilt have a motion to the VV. with respect to Quito . In

a bd -! narme r, every particle of water has its gravity diminished,Pon * tS tilunial motion retarded ; and hepce arises a general mo-lt, or current from E. to W. This is very distinctly perceivedG 0 n? Atlantic and Pacific Oceans . It comes round the Cape ofr ec .| “ope, ranges along the coast of Africa, and then sets di-c om y ? Ver to America , where it meets a similar stream whichit •„ f ' n Py the N. of Europe. Meeting the shores of America ,a i I1ected both to the S. along the coast of Brazil , and to thev‘ on « llie N. American shores, where it forms what is called

° L * tv.—MO. 208 .

the Gulf Stream, (See America, and Mexico, ) because it comesfrom the Gulf of Mexico . This motion is indeed very slow, thisbeing sufficient for the accumulation of seven or eight feet cn thedeep ocean ; but it is not altogether insensible. We may expectdifferences in (lie appearances on the W. shores of Europe andAfrica, and ..on the W. coast of America , irtnu the appearanceson the E. coasts of America and of Asia , for the general currentobstructs the waters from the VV, shores, and sends them to the]*),shores. Also when we compare the wide opening of the N. ex-tremity of the Atlantic Ocean with the narrow opening betweenKamlschalka and America , we should expect differences betweenthe appearances on the VV. coasts of Europe and of Amenta,The observations made during the circumnavigations of CaptainCook and others shew a remarkable difference. Ail 1 along theVV. coast of N. America the inferior tide is observed to he verytrifling, and often is not perceived. In the very same manner,the disturbing forces of the sun and moon form a tide in the fluidair which surrounds this globe, consisting of an elevation and de-pression, which move gradually from E. to VV. Neither doesthis tide ever attain that position with respect to the disturbingplanets, which it would do were the earth at rest cn her axis.Hence arises a motion of the whole air from E. to \Y. and thisis (he principal cause of the trade-winds. They are a little acce-lerated by being lieated, and therefore expanding. They expandmore to the VV. than in the E. because the air expands on thatside into air, which is now cooiing and contracting. These windsvery evidently follow the sun’s motion, tenchng more to the S. orN. as he goes S. or N. Were this motion considerably affectedby the expansion of heated air, we should find (lie air rathercoming northward and southward from the torrid zone, in conse-quence of its expansion in that climate, We repeat it, it is almostsolely produced by the aerial tide, and is necessary for the veryformation of this tide. We cannot perceive the accumulation, itcannot affect the barometer, as many think, because, though theair becomes deeper it is only because it is made lighter bv the gra-vitation to-the sun. Instead of pressing more on the cistern of thebarometer, we imagine that it presses less; because, like theocean, it never attains the height lo whicli it tends. It remainsalways too low for equilibrium, and therefore it should press withless force on the cistern of a barometer. There is an appearanceprecisely simjlar to this in the planet Jupiter . He is surroundedby an atmosphere, which is arranged in zones or belts, probablyowing to climate-differences of the different latitudes, by whicheach seems to have a different kind of sky. Something like thiswill appear lo a spectator in the moon looking at this earth. Thegeneral weather arid appearance of the sky i; considerably diffe-rent in the torrid and temperate zones. Jupiter ’s belts are not ofa constant shape and colour; but there often appear large spots ortracts of clouds, whirl) retain their shape during several revolu-tions of Jupiter round his axis. To judge of his rotation by oneof these, w e should say that he turns round in 9m. 55s. There isalso a brighter spot which is frequently seen, occupying cue cer-tain situation on the body of Jupiter . Tins is surelv adherent tohis body, and is either a bright coloured country, or perhaps a tractof clouds hovering over some volcano. This spot torus roundin 9m. 51js. And thus there is a general current in his atmosphere,from E. to VV. Both the motion of the air and of the water tendto diminish the rotation of the earth round her axis: for theymove slower than the earth, because they are retarded by the lu-minaries. Thev must communicate this retardation to the earth,and must take’from it a quantity of motion precisely equal towhat they want, in order to make up the equilibrated tide. Inall probability this retardation is compensated by other causes; forno retardation can be observed. This would have altered thelength of the year since the time of Hipparchus , giving it asmaller number of days. We see causes of compensation. Thecontinual wasiiing down of soil from the elevated parts of theearth must produce this effect, by communicating to the valleyon which it-is brought to rest, the excess of diurnal velocity whicliit had on the mountain top.

TIDEMAN, Philip, an eminent painter, born at Hamburg,in 1657. He studied under Lairesse, and painted subjects ofallegoryand fabulous history,with great propriety. He died in 1705.

TIDES WALL, or T IDRSVV EI.L, a town of England, inDerbyshire , 22 or 32 miles N. N. W. of Derby, and 158 N. ofLondon . Lon. 1. 46, W. Lat. 53,15, N.

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