Hydrographical Observations from the Danmark Expedition. 373 



responding layers further out in the sea and thus tended to move 

 eastwards, whilst the under layers, as reaction current, had a west- 

 going tendency and mixed with the upper layer on the barrier into 

 Øresund. In the beginning of September the rivers froze over. It 

 may be assumed, that there was now equilibrium for a time between 

 the water-masses near land and out in the sea, so that the east-going 

 movement of the surface water ceased. The fjords then froze and 

 the salinity increased consequently in the upper layers. 



In the course of the winter 1907 — 08 the reverse to what took 

 place in the summer probably occurred, the upper layers in the fjords 

 now increasing in density (both o\\'ing to the formation of ice and 

 the mixing processes) more than the corresponding layers of the al- 

 ready ice-covered sea. Thus, some of the bottom-layer was drawn out. 



From September 1907 to the middle of June 1908 we Ьал^е no 

 observations, but the series LX (middle of June) indicates this 

 increase of the salinity in the surface layer in the course of the 

 winter. 



In the layer — 50 m. the average salinity rose from 31.26 to 

 32.Ö5 (see PI. XX), thus altogether 1.19 p. m. A layer of ice of ca. 

 1 % m. in thickness was formed and if, as already explained, we take 

 the salinity of the ice as 9 p .m., the freezing of a layer of 1.25 m. 

 causes an average increase in salinity of only 0.55 p. m. in the layer 

 from — 50 m. The remaining and greater part of the salinity in- 

 crease, 0.74 p .m., must therefore have been caused by mixing л\11Ь 

 deeper layers. 



On comparing the series LX A and XXV, both taken at the same 

 time of year (middle of June) in 1908 and 1907, we see, that the layer 

 of ice-cold water (i. e. at freezing point) and of nearly homogenous 

 salinity, was only ca. 10 m. deep in 1908 against over 100 m. in 1907, 

 and that the under layer (75 — 150 m.) was considerabley warmer and 

 more saline in 1908 than in 1907. The average salinity was 33.16 

 in 1908 against 32.84 in 1907. 



The characteristic feature of the changes in the mouths of Øre- 

 sund Avas, that at a depth of ca. 50 — 75 m. a mixed layer with almost 

 unchanged temperature and salinity occurred the whole year round, 

 forming the boundary between an upper and lower layer Avith currents 

 flowing in different directions. 



Throughout this mixed layer, under ordinary conditions, a flow 

 of heat passed to the upper layer, which would tend to hinder the 

 formation of ice in the лу1п1ег. 



At the same depth as the mixed layer most of the series show 

 an abrupt transition in density, pгoЛ'ing that a boundary existed 

 here between two layers of different character. 



The mixing processes would naturally tend to even out this dif- 

 ference in density between upper and lower layer; since nevertheless 



