DENSITY OF SEA WATER DENSITY COLOUR AND PHOSPHORESCENCE OF THE SEA Since sea water contains from 3.5 to 4.5 per cent, of solid matters in solution, it is therefore, bulk for bulk, heavier than fresh water. For instance, a cubic foot of fresh water weighs 1,000 oz., but the same quantity of sea water weighs 1,026 on Carefully conducted experiments have been repeat edly made to determine accurately the specific gravity or relative weight of sea water, absolutely fresh water at 62° F. being taken as the standard. The mean of several results gives 1.0275 as the specific gravity of sea water, fresh water being taken as 1. But, as we have said before, the degree of salinity is not exactly the same in every part of the ocean, being greatest in areas subject to active evaporation, and least in those with minimum evaporation, and yet receiving large supplies of fresh water; and the salter the water is, the greater will be its specific gravity, but the difference in various localities is very slight. Thus the specific gravity of the waters of the North Atlantic is said to be 1.0266, and that of the South Atlantic, 1.0267, showing a difference of .0001 only ; that of the North Pacific, 1.0254, of the South Pacific, 1.0265, a difference of only .0011 ; that of the Baltic, 1.0086, of the Red Sea, 1.0286, a difference of .02 ; the Mediterranean, 1.0289 ; and the Indian Ocean, 1.0263. So that the average difference in the specific gravities of the Atlantic and Pacific Oceans is .0007, or, in other words, the Atlantic is slightly salter than the Pacific. On the other hand, the difference between the Baltic and the Mediterranean is more marked, being .0203, that is, the water of the Mediterranean is one-fiftieth part salter than those of the Baltic; or, in other words, 100 lbs. of Mediterranean water would contain .08 lbs, more saline matters than the same quantity of Baltic water. The difference in this case exceeds the general variation, and a proportion of only 1.28 oz. per 100 lbs. might be looked upon as practically too small to be noticed, were it not that the least difference is the immediate cause of an incessant interchange of the particles of water of different densities; and thus, as we shall again point out more fully, the vertical, and probably in a large measure the horizontal, circulation of the waters of the ocean, immediately depend on an infinitesimal difference in the density, whether arising from a varying degree of saltness or temperature, of different parts of the ocean.
Fresh water is specifically lighter than salt water, and as enormous quantities of fresh water are constantly poured into the sea by rivers, or precipitated on its surface as rain, the surface water of the sea will be found to be gene rally fresher than that at great depths, inasmuch as the lighter fresh water will " float " on the surface for a considerable time before it is thoroughly mixed with the underlying salt water.
Generally speaking, then, the specific gravity of sea water at the surface will be somewhat less than that at great depths. But besides this, water is slightly compressible, being at a depth of 1000 feet compressed wiz- of its bulk. The pressure of the superincumbent water at a depth of one mile has been calcu lated at 2000 lbs. per square inch, and this pressure increases at a uniform rate to the lowest depths. But, strangely enough, the density does not increase uniformly with the depth, and in some localities the surface water is actually denser than that at the bottom. Thus the specific gravity of the surface water at a point in the north Atlantic, 28' N. lat., and 44' W.
long., was found to be 1.028; while at the bottom, at a depth of 8580 feet, it was only 1.0269—the surface water showing excess in density of The greater density in this case was most probably due to temporary causes, such as excessive evaporation and limited precipitation at the time of the observation ; but in certain areas the same causes are so con stant in their operation, that the surface water is permanently denser than that at the bottom.
The greatest density of fresh water is at 39.2° F. ; fur ther reduction in its temperature causes it to expand until it reaches the freezing point, 32° F. But salt water does not ex pand until it reaches the freezing point, so that its point of greatest density nearly corresponds to its freezing point. This point is several degrees lower than that of fresh water, being F., or even lower, under certain conditions. And since fresh water only is converted into ice, the freezing of the surface waters of the polar seas is always accompanied by an increase in the saltness of the water immediately beneath the icy crust: an effect exactly the same, but produced by very different means, as that of excessive evaporation. So that excessive cold as well as excessive heat causes an increase of the salinity of the sur face-water—the former by withdrawing the fresh water in the form of ice, the latter removing the fresh water in the shape of vapour. In both cases the salts, originally dissolved in the displaced portion of the water, is left behind, and the denser surface water gradually sinks, and is replaced by lighter water from below, which is again subjected to the same process. This difference in the density of surface and deep water origin ates and maintains a constant "vertical circulation" of the waters of the ocean. But, as we shall explain further on, although a difference of density alone might be a sufficient cause to produce a vertical circulation at any one point, the original movements of the particles of water consequent on a difference in density, due to the saltness of the water, are affected by the constantly varying temperature from the equator to the poles, and from the surface to the bottom.