120 PACIFIC OCEAN none at considerable depths, for the isotherm of 40 is constantly between 300 and 400 fathoms, and also that depth alone deter mines the bottom temperature in the open ocean, the coldest water occurring as a matter of fact under the equator in the deepest troughs open to the south. Density Density of the Water. The specific gravity of ocean of the water is an index of its salinity, since the researches of water, various chemists, foremost amongst whom are Forchham- mer and Dittmar, have shown conclusively that the per centage composition of the salts in sea water is the same in all parts of the ocean, so far at least as regards the principal constituents. Mr J. Y. Buchanan made continu ous observations on the specific gravity of sea water during the whole voyage of the "Challenger," and has published a very valuable paper on the distribution of salt in the ocean in the "Challenger" Reports (Phys. Chem. Chall. Exp., Plate II. vol. i. part ii.). The chart in Plate II. showing the geogra- chart. phical distribution of surface density is copied from that paper. The percentage of total salts in sea water, as deduced from the specific gravity, is, according to Buchanan and Dittmar Density 1 025 1 026 1 027 1 028 Percentage 3 3765 3 5049 3 6343 37637 The density of the water in different parts of the ocean must obviously change to a certain extent with the season; and it is not only the surface density that is affected in this way ; any cause which promotes evaporation tends to increase the salinity of surface water, while any con ditions that effect condensation of aqueous vapour produce dilution. For instance, in the China Sea during the month of November, at the end of the south-west monsoon, which is a moist wind accompanied by much rain, the specific gravity observed was 1 0251 8, and two months later, after the dry north-east monsoon had been blowing for some time, evaporation had proceeded so far that the specific gravity had risen to 1 02534. The climate is the principal factor in determining surface salinity, and the causes which produce well-marked climatic conditions have an equally apparent effect on the density of the water. Thus there are two zones of comparatively high density encircl ing the globe in the region of the north east and south-east trade winds, which are dry and promote rapid evaporation ; and similarly the region of calms and rain between the trades is distinguished by the low specific gravity of the water. North and south of these areas there are two zones where the salinity maintains a mean value, in consequence of there being a balance between evaporation and con densation ; and round the poles there are areas of concentration brought about by the freezing of the sea water and the separation of salt, which of course increases the salinity of the water remaining unfrozen. The distribution of density differs considerably in the two great oceans. In the Atlantic there are two areas of high specific gravity, one in the north, the other in the south ; while in the Pacific there is only one, situated in the southern division of the ocean in the neighbourhood of the Society Islands. It is neither so large as those of the Atlantic, nor has it so high a specific gravity. The density of the concentration areas in the Atlantic, taking pure water at 4 C. as unity, is 1 02750; that in the saltest portion of the Pacific is only 1 02700. In the North Pacific the salinity i.s less than in the South, and it distribution is much more uniform. The density in this region never exceeds 1-02650, and the minimum, in the rainy region of the equatorial counter current, i.s as low as 1 02485. The South Pacific has water of a relatively high density, its maximum being 1-02750. The water of the seas of the Eastern Archipelago, in the western basin of the Pacific, although exposed to the full force of an equatorial sun, and possessed of a very high surface temperature, is yet surprisingly fresh. The specific gravity varies considerably with the season, but the aver age for the year over the greater part of these seas is under 1 02550 ; and there is a large area surrounding the islands of Java and Sumatra where the dilution is greater, the hydrometer only indicating 1 02500. The weak salinity of these waters is largely to be attributed to the extreme humidity of the atmosphere, the frequent and heavy rains, and the fact that so many lofty and exten sive islands, where the annual rainfall rises above 200 inches, drain into the seas. Water of such a degree of dilution is not met with anywhere else, except near the mouths of rivers and in the vicinity of melting ice, and, as a temporary phenomenon, after prolonged rain in the tropics. In regions where there is decided and continuous con centration in progress, the specific gravity of the water is greatest at the surface and decreases as the depth increases, down to about 800 or 1000 fathoms, after which the density increases slowly with the depth until the bottom is reached. The density of the bottom water of the Pacific is almost the same everywhere ; it only varies from 1 02570 to 1 02590; and the same value holds for the South Atlantic. The North Atlantic has denser water at the bottom, varying from 1 02G16 to 1-02632. In those regions where the surface water is being constantly diluted, as is the case in the equatorial belt of calms, the density increases with the depth down to between 50 and 100 fathoms, where there is a maximum, from which the density diminishes, as in the other case, to about 1000 fathoms, and afterwards increases slowly down to the bottom. There is a striking resemblance between the direction of the isohalsines, or lines of equal salinity, and of the isothermals ; but the parallelism breaks down, of course, in the case of a subsurface maximum. Depth.- For a long time the opinion that the Pacific Depth, was a comparatively shallow ocean was entertained by Plate III. geographers, and it is only the recent soundings of the "Challenger," " Tuscarora," "Gazelle," and other survey ing ships that have succeeded in dispelling the illusion. It is now known that the average depth of the Pacific is greater than that of the Atlantic, and that areas of deeper water occur in it than in any other part of the globe. A line running along the western shores of the two Americas and along the eastern shores of the Asiatic continent more or less closely follows a great circle of the globe. On the one side of this line there are the continental masses of the Americas and of Europe and Asia, with an average height of about 800 feet above the level of the sea ; and on the other side the vast oceanic depression of the Pacific, with an average depth of about 2500 fathoms. The average level of the continental area may thus be regarded as about three miles above the Pacific depression. The attempt to divide the ocean into sharply defined basins is more or less unsatisfactory; and for the considera tion of the depth it is better to view the Pacific as marked off into two portions by an imaginary line passing through Honolulu and Tahiti, on the meridian of 150 W. The eastern half is remarkable for the comparative absence of islands and the uniform nature of its depth. With the exception of the narrow strip of shallow water surrounding the Aleutian Islands and running along the American coast, the sounding line shows an average depth of from 2000 to 3000 fathoms undiversified by remarkable elevations or depressions, between the northern limit of the ocean and 30 S. lat. There is a great submarine plateau extending from the Patagonian coast (in 76 W. long.) in a westerly direction to 120 W. long., which rises to between 2000 and 1000 fathoms of the surface. This elevated area diminishes in breadth as it proceeds westward, but it is supposed by some authorities to be
