PHYSICAL GEOGRAPHY. 



there to form a similar protuberance or high wave. 

 In the one case, the water is drawn directly up or 

 towards the moon (M) ; in the other, the water is 



in some shape left behind by the land being pulled 

 away from it. In both a similar effect is produced : 

 two tides (t, /) are caused at opposite extremities 

 of the earth. Where the higher part of either of 

 these great billows strikes our coasts, we have the 

 phenomenon of high-water; and when the lower 

 touches us, it is low-water. Each of the waves is 

 brought over any given place in the circumference 

 of the earth in twenty-four hours, so as to cause 

 high-water twice a day. The sun exerts a far 

 greater attractive influence on the earth than the 

 moon does ; but from the great distance of that 

 luminary, the difference of that attractive force on 

 different parts of the globe is much less, and there- 

 fore the effect in raising tides is comparatively 

 small. But when this minor influence of the sun 

 coincides with that of the moon, or acts in the same 

 line of attraction (M/), we perceive a marked in- 

 crease in the tides ; on such occasions we have what 

 are called spring or large tides. When the solar 

 and lunar attractions act in opposition, we have neap 

 or small tides. The spring-tides happen twice a 

 month, when the moon is at full and change ; and 

 the neap when the moon is in the middle of its orbit 

 between those two points. A tide requires six hours 

 to rise which it does by small impulses or ripplings 

 of the water on the shore and six hours to ebb or 

 fall ; but every successive high-water is from twenty 

 to twenty-seven minutes later than the preceding, or, 

 on an average, about fifty minutes for two tides, in 

 consequence of the earth requiring that time above 

 the twenty-four hours to bring any given point 

 again beneath the moon. The tides are thus re- 

 tarded by the same reason that makes the moon 

 rise fifty minutes later every day. It is evident 

 that the tides will be greatest at that point of the 

 earth's surface which is nearest to the moon, or 

 where the latter is vertical. She is so between the 

 tropics ; and accordingly the tides are there great- 

 est, and they diminish as we approach either pole. 

 It is further to be remarked that the moon does 

 not anywhere draw up the tides immediately. In 

 consequence of the law of inertia and of fluid 

 friction, the tidal wave lags behind the moon. 

 Moreover, in consequence of all the great seas and 

 oceans forming, as we have seen, only one sheet of 

 water variously distributed, the ebb and flow in 

 each depend not on its own proper tide, but are the 

 result of the combination of that tide with currents 

 mingling with it from tides of other seas a result 

 depending upon inequalities of sea-bottom, the 

 configuration of its coasts, their inclination under 

 water, the size and direction of the channel which 

 connects it with other seas, and occasionally upon 

 winds and currents which are not tidal. So much 

 do these circumstances affect the astronomical or 

 primary tidal wave, that while it rises in the 



expanse of the Pacific to one or two feet ^ 

 derived wave often rises in confined or obstructe 

 seas to elevations of thirty, fifty, or even a hundred 



feet ! Inland expanses of water, like the Baltic, 

 Mediterranean, and Caspian Seas, and the lakes 

 of North America, have no perceptible tides. 



Besides being affected by the regular motion of 

 the tides, the ocean is pervaded by a system of 

 currents, mostly constant, which has been com- 

 pared to the circulation of the blood. These cur- 

 rents play a most important part in modifying 

 the climates of different regions. As respects 

 the causes of oceanic currents, much remains to 

 be cleared up ; but some of the leading causes 

 seem well established. The two prime movers 

 are differences of temperature and prevalent 

 winds. Sea-water does not freeze until it is 

 cooled down to about 28 ; and, unlike fresh 

 water, it continues to grow heavier down to that 

 point. The effect of the intense cold of the polar 

 regions is thus to cause a constant sinking down 

 of the surface, and to establish a current of ice- 

 cold water along the bottom towards the equator ; 

 while, to supply the place of what sinks down, an 

 in-draught or northward flow takes place on the 

 surface, which brings the warm water of the 

 temperate and tropical regions towards the poles. 

 This is the general theory of the vertical circu- 

 lation of the ocean a circulation which might 

 almost be assumed from the well-known laws of 

 the flow of liquids, and which recent observations 

 have established as a fact. The general prevalence 

 of cold currents along the bed of the ocean from 

 the poles to the equator is now beyond dispute. 

 Motion once thus begun, however, is differently 

 modified in each locality by the shape of the coasts, 

 by prevalent winds, and other circumstances. But 

 one cause which modifies all currents that tend 

 either north or south, is the daily rotation of the 

 earth. At the equator, any spot on the surface is 

 moving eastward at the rate of icoo miles an hour ; 

 at 6o c north latitude, the velocity is only one half. 

 Thus, the water of a current starting from the 

 equator northward, is constantly coming to places 

 where the bottom under it has less and less east- 

 ward velocity. But, by the law of inertia, the 

 water tends to retain the same velocity eastward 

 with which it started, and thus it moves to the east 

 of north shooting ahead, as it were, of the bottom 

 over which it is flowing, as a rider does whose 

 horse slackens his pace. The contrary happens 

 to a stream flowing from north to south. In this 

 case, the eastward motion or motal inertia of the 

 water is too slow for the parts of the bottom to 

 which it successively comes ; the bottom slips in 

 a manner from under it, and it falls to west of 

 south. This, in combination with the action of 

 opposing coasts, accounts for the circular sweep 

 which many of the currents make, returning partly 

 into themselves. 



Different in origin from this vertical circulation, 

 though partly mixed up with it, is the horizontal 

 circulation caused by prevalent winds. The best 

 example of this is the Equatorial Current, which 

 sets from the west coast of Africa to the east 

 coast of Brazil, and which is owing to the action 

 of the trade-winds. Currents caused by winds 

 are always shallow, and their rate of motion 

 seldom exceeds half a mile an hour : they are 

 called ' drift-currents,' in opposition to the deeper- 

 seated 'stream-currents.' In order to feed this 

 westerly equatorial current, there spring up two 

 currents the one from the north 



, 

 along the west coast of Portugal and Morocco, 



61 



