CHAMBERS'S INFORMATION FOR THE PEOPLE. 



terra firma. Proceeding west from Ireland, the 

 bottom descends in a succession of steps or 

 terraces, sometimes of immense extent. The 

 descent from one plateau to another is sometimes 

 by steep cliffs 9000 feet deep. The greatest 

 depth is some distance south of the great bank ol 

 Newfoundland, where there is a basin-shaped 

 depression 1000 miles in length, with soundings 

 of about 30,000 feet, or above five miles and a 

 half. The bottom of the Pacific is less known : 

 soundings of 40,000 feet have been obtained. 



The temperature of the sea, where it is not 

 affected by currents from a warmer or colder 

 region, necessarily corresponds to that of the air 

 above it ; but this is true only of the water at 

 and near the surface. In the Mediterranean, for 

 example, the surface temperature ranges from 8o r 

 in summer to 54 or 55 in winter. But this 

 excess of summer heating does not extend beyond 

 a depth of 100 fathoms : below this, the tempera- 

 ture of 54 or 55 is maintained constantly at all 

 depths ; and this temperature is that of the crust 

 of the earth in that region. In the Atlantic, in 

 the same latitude, the temperature of the upper 

 stratum of 100 fathoms does not essentially differ 

 from that in the Mediterranean ; but at greater 

 depths, the temperature sinks to 49, 40, 38, 36, 

 and in some places to the freezing-point of fresh 

 water, and even below it (29'6). Recent obser- 

 vations tend to shew that an almost glacial cold- 

 ness prevails over the whole deep-sea bed in both 

 hemispheres, extending even to the tropics. The 

 cause of this phenomenon will be explained when 

 we come to speak of the currents of the ocean. 



The sea consists of salt water, as already ad- 

 verted to, and from its continual motion, under 

 the influence of currents and waves, preserves, 

 generally speaking, uniform saltness. Under 

 special circumstances, however, we find the salt- 

 ness increased, as by the excess of evaporation 

 over the fresh-water influx in the Mediterranean 

 and Red Seas, and about the northern and 

 southern limits of the tropical belt ; and 

 decreased, by the contrary cause, in the Sea of 

 Azof, Black Sea, Baltic Sea, and in the polar 

 regions. The origin of the saltness of the sea is 

 sufficiently accounted for when we consider that 

 the chloride of sodium and other soluble salts 

 which form constituent ingredients of the globe, 

 are being constantly washed out of the soil and 

 rocks by rain and springs, and carried down by 

 the rivers ; and as the evaporation which feeds 

 the rivers carries none of the dissolved matter 

 back to the land, the tendency is to accumulate 

 in the sea. The principal ingredients found in 

 sea-water are chloride of sodium, or common salt, 

 together with salts of magnesia and lime. The 

 mean quantity of salts of all kinds in the ocean is 

 34-4 parts in icoo. In some parts of the Mediter- 

 ranean the salinity is as high as 39-26, and of the 

 Red Sea 43 ; while in the Baltic and Black Seas, 

 it varies from 18 to 12. Of the mean of 34-4 

 parts, about 24 are due to chloride of sodium, 4 

 to chloride of magnesium, nearly as much to sul- 

 phate of soda, one part to carbonate of lime, 

 and -25 or one part in 4000 to silica. Up- 

 wards of thirty different elementary substances 

 have been already detected in sea-water, and spec- 

 trum analysis may yet reveal more ; but, except 

 those named above, they exist mostly in exceed- 

 ingly minute proportions, so that it is only in the 



60 



analysis of sea-weeds, marine animals, and the 

 stony matters deposited in marine boilers, that 

 their presence can be detected. But when we 

 consider the vastness of the ocean, the absolute 

 quantity of even the minutest ingredient must 

 be enormous. The silver dissolved in the sea 

 would far transcend ' the wealth of Ind.' 



The specific gravity of the water of the ocean 

 varies, of course, with the salinity ; the average 

 is 1-0272. Another effect of the salinity is that, 

 while fresh water freezes at 32 F., the water of 

 the ocean requires to be reduced to 28 ; and the 

 ice then formed is porous and full of a briny fluid 

 the solid ice in fact is fresh. 



The colour and phosphorescence of the ocean 

 are the next sensible properties requiring atten- 

 tion. When examined in small quantities, sea- 

 water is colourless ; but when viewed in the mass 

 in the wide ocean, it appears to be of an azure or 

 blue tint. The cause of this generally blue colour 

 has not been as yet clearly explained; it would 

 seem that the blue rays of light are more readily 

 reflected from masses of a transparent fluid. 

 While there can be no doubt that the ocean is 

 generally of a blue colour, it is equally certain that 

 there are many portions of sea in which a different 

 hue appears. The causes of these exceptions from 

 the rule seem to be of various kinds. Frequently 

 the ordinary colour of the sea is affected by the 

 admixture of foreign substances, these being some- 

 times of a living and organic nature, and some- 

 times not. Another class of cases in which 

 the ocean appears to be tinged with a peculiar 

 colour, is referable to the reflection of rays of light 

 from the bed or bottom ; and hence, in shallow 

 and clear seas, the colour of the ground is a main 

 cause of any particular tint which the water may 

 there assume. 



The phosphorescence of the ocean, described in 

 such glowing terms by almost every voyager in 

 tropical seas, is now satisfactorily ascertained to 

 arise sometimes from the presence of infusorial 

 animalcules, and at others from the decomposi- 

 tion of vegetable and animal matter. Similar 

 phenomena, arising from similar causes, exist on 

 land the glowworm, fire-fly, certain fungi, putrid 

 fish, &c. and their appearance in the one ele- 

 ment need not excite greater surprise than their 

 exhibition in the other. 



TIDES CURRENTS WAVES. 



The waters of the ocean are subject to various 

 motions and fluctuations, such as tides, currents, 

 whirlpools, waves. That regular ebb and flow 

 known by the name of tides, and which confers 

 on the ocean one of its most interesting features, 

 is caused by the attraction of the sun and moon. 

 By the universal law of gravitation, all masses of 

 matter have a tendency to be attracted or drawn 

 towards each other. The moon, therefore, as a 

 mass of matter, in passing round the earth, has a 

 tendency to draw the earth after it, or out of its 

 natural relative position ; and it really does so to 

 a small extent. As it passes round, it draws up 

 the waters in a protuberance, or, in common 

 language, draws a huge wave after it. But it also 

 draws the whole solid globe though to a less 

 extent than the water immediately under it and 

 so causes the opposite side of the globe to be 

 drawn away from the ocean, leaving the waters 



