Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/313

Rh water to be discharged is then at its greatest. It some times happens, especially about the autumnal equinox, that a N.W. gale concurs with a high tide in the Skager Rack to drive its water towards the Baltic, causing it to overflow the lower portions of some of the Danish islands. If, then, a southerly wind should carry this water onwards into the Gulf of Finland, the check which it gives to the downflow of the Neva produces disastrous inundations at St Peters burg. (3.) The influence of atmospheric pressure upon the height of the water in the Baltic is very remarkable. It had long been noticed that its level occasionally rises even as much as 3 feet without any apparent cause, and maintains itself at that height, sometimes only for a few days, but occasionally for several weeks together, and this at all seasons. Schultdn, having observed that such elevations of level are preceded by a fall of the barometer, and that when the barometer rises again the water subsides, was led to recognise the dependence of these changes upon converse changes in atmospheric pressure ; and this reference was confirmed by observation of the constant proportion borne by one to the other. A similar con sequence of variation in atmospheric pressure has been observed in the Mediterranean (see ) ; and it has also been noticed in England as a disturbing ele ment in modifying the height of the tides.

Salinity of Baltic Water.—As might be expected from what has been already stated, the proportion of salt in the water of the Baltic is very much below that of oceanic water, and varies greatly at different seasons. In the Gulf of Bothnia, at the time the river-flow is greatest, the surface water is often so little salt as to be quite drinkable, its sp. gr. having been found as low as 1 - 004. But it is said to contain at Christmas six times as much salt as at midsummer, showing that when the river supply is at its lowest, its place is taken by a reflux of salt water from the outside ocean. In the Baltic proper there is a very decided difference in salinity between the upper and the lower stratum ; the less saline water of the surface flowing towards the outlet over the more saline water beneath, just as the fresh-water current of a great river runs out to sea, even far beyond the sight of land. Thus the proportion of salt in 1000 parts of a sample of surface- water taken near Stockholm being 5 &quot;9 19, that of bottom- water, brought up from 120 fathoms, was 7*182 ; and in like manner the proportion of salt in s?/ace-water at the entrance of the Gulf of Finland being 3 552, that of bottom-vraier at 30 fathoms depth was 4 - 921, the proportion of salt in North Sea water averaging 32 - 82 3 parts in 1000. Nearer the outlet the proportion of salt is greater alike in surface and in bottom-water. From the careful and systematic observations of Dr Meyer (orj. dt. it appears that the sp. gr. of the surface-stratum at Kiel ranges between about l OOS2 in summer and 1 0142 in winter, the latter showing somewhat above half the quantity of salt contained in ordinary sea-water. But if the direction of the prevalent winds during the autumn be such as to maintain a strong surface out-current, and consequently (as will presently appear) a very strong inward under-current, as happens in some years, the maximum of salinity will present itself at that season. The sp. gr. of the deeper stratum ranges at Kiel from 1-0145 to 1-0190; at Helsingor on the Sound from 1-0190 to 1-0220 ; and at Korsor on the Great Belt from 1 0180 to 1 0243 ; thus showing it to be principally composed of North Sea water, whose sp. gr. may be taken as 1-0264.

Currents in the Baltic Straits.—The results of observation of the movements of the upper and under strata of water in the Baltic Straits, strongly confirm the doctrine else where enunciated (see ) in regard to the potency of slight differences of downward pressure in the production of under-currents. The prevalent movement of the upper stratum in the Baltic Straits is outward ; and this concurs with the low salinity of Baltic water to indicate that it is partly an overflmo current, produced by the excess of river supply over loss by evaporation, which tends to raise its level. But even when this outward surface-current is strong, there is usually an inward under-current of North Sea water, carrying back into the basin of the Baltic a large proportion of the salt which would otherwise be lost to it ; and the existence of this under-current, which has been abundantly established by experimental inquiries, as well as by the observations of divers, is exactly what theory would lead us to predict. For if two columns of water of the same height, but differing in specific gravity, be made to communicate with each other aliie at the surface and at the bottom, the lower part of the heavier column, having a greater lateral pressure, will flow towards the lighter, thus tending to produce an elevation of level in the latter, which will rectify itself by a surface-flow in the opposite direction; and thus a vertical circulation will be maintained, as long as the causes which maintain the difference of salinity remain in operation. Now, as the salinity in the oceanic column may be regarded as practically constant, whilst the salinity of the Baltic column, though not uniform, is kept down by the influx of river- water to a much lower degree, this difference will always exist to a greater or less amount. When, however, the height of the Baltic column is so much raised either by the excess of its fresh-water supply, or by the reversal of the surface-current by the agency of wind that the downward pressure of its less saline water exceeds that of the more saline water of the North Sea column, the under-current will be brought to a stand, or its direction will be reversed. Thus it is that when the outward movement of the upper stratum depends rather upon the prevalent winds (as is usually the case during autumn and winter) than upon the elevation of its level within the basin, the inward under-current which supplies its place is strongest and most constant. And it is by this means, much more than by the occasional reversal of the surface-current, that salt is carried back into the Baltic, as is proved by the close correspondence shown by Dr Meyer s observations to exist between the predominance of the inward under-current and the elevation of the sp. gr. of the surface-water of the Baltic. On the other hand, it is during the spring and summer months, when the outward movement of the upper stratum is rather an overflow- current, and the salinity of the surface-water is the lowest, that the under-current sets less strongly and less constantly inward.

Zoology.—The fauna of the Baltic may be regarded as that of a large estuary, having a narrow communication with the sea, its marine inhabitants being such as can adapt themselves to considerable variations in the salinity of its water. Whales rarely enter the Baltic ; but porpoises fre quent the neighbourhood of the Danish islands. Seals are obtained in considerable numbers at the breaking up of the ice around Gottland and the Aland Isles. The salmon is among the most abundant fishes of the Baltic proper, ascending its rivers from April to June ; and salmon-trout are caught in some of its bays. The portion of the Baltic in the neighbourhood of the Danish islands is frequented by various species of Gadidce, which do not range further east. In the 14th and loth centuries there was a consider able herring-fishery within the Sound and along the coast of Scania (the southern portion of Sweden) ; but this fish seems to have latterly quite deserted the Baltic, and rarely shows itself even in the Cattegat. On the eastern coast of Sweden, on the other hand, and in the Gulf of Bothnia, a fish called the stromling, which is nearly allied to the herring, being chiefly distinguished by its small size, is 