The general depth of the Baltic is 60 fathoms :1 but, towards its south-east extremity, and nearly ] in the middle, are two spots, with 110 and 115 fa thoms of water. From the east mouth of the Sound to Bornholm, the depth varies from 9 to 30 fathoms; from thence to Stockholm from 15 to 50 ; a little south of Lindo it is 60; and among the Aland islands from 60 to 110.
It was long a generally received opinion, that the waters of the Baltic were considerably more elevat ed than those of the German Ocean, and that they w,Fre gradually diminishing. The first opinion seems to have rested entirely on the fact of a constant cur rent setting out of the Baltic. To this it is ascribed by Vice. Admiral Nordenancker, who was President of the Swedish Academy in 1792. In a paper pub lished in their Transactions, he maintains that, from observations made at different periods, the height of the waters of the Baltic was diminishing at the rate of about four and a half lines annually. Celsius, a learned Swede, towards the middle of the last cen tury, advances the same hypothesis, and, from obser vations made on the coasts of the Baltic, he estimat ed the diminution at 45 inches in every hundred years. This hypothesis was supported by Linnwus, who founded on it a theory of the earth ; but the chief facts brought forward in support of this opi nion, viz. marks on several rocks in the Gulf of Both nia, and the remains of vessels found at considerable distances from the present shores, by no means war rant it. M. Otto, in his physical observations on this sea,'has suggested another theory to account for its apparent decrease. He supposes that, instead of . really subsiding, it may be only shifting its position, and gaining in one quarter what it loses in another ; and this he ascribes to the large and rapid rivers which carry along with them an immense quantity of earth and sand ; and thus the beds at their. mouths are raised, and their banks extended towards the sea. proved. But recent observations made at the locks of the ca nal of Holstein prove, that the levels of the 'Baltic and the ocean are at present generally the same, and that the trifling differences which may be occasionally observed, are owing to accidental and temporary causes.
Hence we may infer, that the constant current setting out of the Baltic, is solely owing to the abundance of the waters which it receives from its rivers.
It is generally believed that there are no tides in This, however, is not strictly correct. There are sensible .tides in the Scagerac ; they begin to diminish in the Cattegat ; are very trifling in the Sound and Belts ; and in the Baltic, properly so called, are scarcely, if at all, perceptible. There are, however, irregular variations in the level of the waters of the Baltic, which bear some resemblance to tides. These elevations generally occur in au tumn, when the weather threatens rain ; they last sometimes a few days, sometimes several weeks. The maximum rise is three feet and a half; and the low shores Are occasionally inundated. They also ren der the fresh-water lakes, which communicate with the sea, brackish. In the Gulf of Bothnia, the fall of the waters is usually succeeded by north winds ; whereas, near Stockholm, these winds usually follow the elevation. M. Kraft, who was Professor of Ex perimental Philosophy in the Imperial Academy of Sciences at Petersburgh, in his treatise on the in undations of the Neva at the autumnal equinox, observes, that three or four days before or after the full or new moon, a violent north-west wind drives the waters of the Northern Ocean, during the influx of the tide, into the Baltic, and is accompa nied, or immediately succeeded, by a south wind in that sea and the Gulf of Finland. By Schultens, a learned Swede, who paid particular attention to the physical geography of the Baltic, the irregular ele vations of this sea are attributed to the state of the atmosphere. He observed, that when the waters are about to rise, the barometer falls, and when they are about to fall, it rises. Hence he inferred, that the un equal pressure of the atmosphere on different por tions of the water, deranged the level of the waters. The difference between the greatest and the least rise in the barometer in the northern parts of Europe _ is two and a half inches, which answers to three and a half feet of water, or the difference of the elevation of the waters at their extremes.