99 



PHYSICAL GEOGRAPHY— THE OCEAN. 



from the surface radiating heat, and cooled by 

 the contact of the atmosphere, could acquire in 

 a tropical region. In the Mediterranean Sea, 

 as Arago acutely observes, a corresponding 

 great depression of temperature in the inferior 

 strata is only not observed, because the influx 

 of the deep polar stream by the Straits of Gib- 

 raltar, through which the Atlantic is flowing 

 from west to east, is encountered by a west- 

 ward under-current of the Mediterranean to- 

 wards the Atlantic. 



The fluid-covering of our planet, equalizing 

 and tempering climates in general, where it is 

 not intersected by pelagic currents of colder or 

 warmer water, and far from the coasts of trop- 

 ical countries, particularly between 10° north 

 and 10° south latitude, may be said to exhibit 

 a truly wonderful equality and steadiness of 

 temperature over areas that are thousands of 

 square miles in extent(333). It has, therefore, 

 been said with reason(^^*), that a long- contin- 

 ued and careful investigation of the thermal 

 relations of the tropical seas would give us in- 

 formation in the simplest manner on the grand 

 and much discussed problem of the constancy 

 of climates, and of the temperature of the earth. 

 Great revolutions in the luminous disc of the 

 sun, were they of long continuance, would be 

 simultaneously reflected in the altered mean 

 temperature of the sea still more certainly than 

 in the mean temperature of the land. 



The zones in which the maxima of density 

 (saline contents) and temperature lie, do not 

 coincide with the equator. The two maxima 

 are distinct from one another, and the warmest 

 water appears to form two not completely par- 

 allel ba.nds to the north and south of the geo- 

 graphical equator. The maximum of saline 

 contents was found by Lenz, in his voyage 

 round the world, in the Pacific, in the two par- 

 allels of 22° north and 17° south latitude. The 

 zone of least density, again, was found to lie 

 a few degrees to the south of the line. In the 

 region of the Calms, the heat of the sun can- 

 not occasion any great amount of evaporation, 

 because a stratum of air saturated with saline 

 vapour there sleeps unmoved and unrenewed 

 upon the surface of the ocean. 



The surface of all the seas that communi- 

 cate one with another, must be regarded as 

 generally perfectly equal in respect of mean 

 elevation. Local causes, mostly prevailing 

 winds and currents, have, however, in particu- 

 lar extensively land-locked seas — the Red Sea, 

 for example, produced permanent, though still 

 inconsiderable differences of level. At the isth- 

 mus of Suez the level of the Red Sea is from 24 

 to 36 feet above that of the Mediterranean at 

 diflferent hours of the day. The form of the ca- 

 nal, (the Straits of Babelmandel), by which the 

 Indian Ocean communicates with the Red Sea, 

 being such, that the waters find a readier ac- 

 cess than outlet, appears to assist in producing 

 this remarkable permanent superior elevation 

 of the surface of the Red Sea, which was al- 

 ready known to the Ancients(^35). The admi- 

 rable geodetical operations of Coraboeuf and 

 Delcros along the chain of the Pyrenees, have 

 shown that there is no appreciable difference 

 in the surface of equilibrium, in the sea-level, 

 on the north coast of Holland and at Marseilles, 

 of the ocean and the Mediterranean("^). 



Disturbances of the Equilibrium and motions 

 of the mass of waters consequent on these, 

 sometimes irregular and transient, depending 

 on winds and producing Waves which in the 

 open ocean and far from land mount during a 

 storm to a height of 35 feet and more ; in oth- 

 er instances, regular and periodical, occasioned 

 by the position and attraction of the sun and 

 moon — the Tides ; in still other instances, per- 

 manent, but of unequal force, as Oceanic cur- 

 rents. The phenomena of ebb and flow, which 

 extend over every sea with the exception of 

 those that are very small and much land-locked, 

 in which the tidal wave is either little or not 

 at all observable, are completely explained by 

 the Newtonian natural philosophy, i. e. referred 

 to the circle of necessary effects. Each of 

 these periodically recurring oscillations of the 

 ocean, is somewhat longer than half a day. In 

 the open ocean they scarcely rise to the extent 

 of a few feet ; but in consequence of the posi- 

 tion and configuration of coasts and estuaries 

 which meet the coming tidal wave they rise in 

 some places to extraordinary heights — in St. 

 Malo to 50 feet, and in Acadia, Nova-Scotia, 

 to from 65 to 70 feet. " Under the supposition 

 that the depth of the ocean is inconsiderable 

 when contrasted with the semi-diameter of the 

 earth, the analysis of the great geometrician 

 Laplace, has shown how the stability in the 

 equilibrium of the ocean requires that the dens- 

 ity of its fluid should be less than the mean 

 density of the earth." And indeed,%s we have 

 seen above, the density of the oarth is five- 

 times greater than that of water. The high 

 lands of the earth, therefore, can never be over- 

 flowed, and the remains of marine animals 

 found on mountains can by no means have been 

 brought into such situations by former floods 

 or deluges produced by the position of the sun 

 and moon(3"). It is no trifling tribute to analy- 

 sis, which in the unscientific circles of society 

 is presumptuously held so cheap, that Laplace's 

 perfected Theory of the Tides has made it pos- 

 sible to predict in our astronomical ephemerides 

 or nautical almanacks, the height of the spring- 

 tide to be expected at each new and full moon, 

 and so to forewarn the inhabitants of the coasts 

 of the increased danger with which they are 

 threatened at these seasons, particularly when 

 the moon is in her perigee. 



Oceanic currents, which exercise so consid- 

 erable an influence on the intercourse of na- 

 tions and on the climatic relations of coasts, 

 are almost simultaneously dependent on a mul- 

 titude of very dissimilar, now greater, now ap- 

 parently more insignificant causes. To the 

 number of these belong : the progressive time 

 of appearance of the ebb and flow of the tidal 

 wave in its course round the world ; the dura- 

 tion and force of prevailing winds ; the density 

 and sp^fic gravity of the watery particles 

 modified under different parallels of latitude by 

 their temperature and saline impregnations(^^*) ; 

 the horary variations of the atmospheric press- 

 ure, which proceed successively from east to 

 west with such regularity within the tropics. 

 The currents of the ocean present this remark- 

 able spectacle : that they cross it of definite 

 breadths in different directions, in the manner 

 of rivers, neighbouring unmoved watery strata, 

 forming the banks, as it were, of these streams. 



