_ IX — PETTERSSON-INTRODUCTORY 



Two different branches of the Atlantic Stream enter the North Sea, one round S""ci'« «f *>= 

 the Orkneys and Shetlands to the north of Scotland, the other through the Eng- in tue North se» 

 lish Channel. This can be seen from the surface-chart. The Dogger Bank and 

 the Fisher Banks form a kind of water-shed ^ between the two hydrographical 

 regions. The isobathe of 80 m. divides the region of the banks from the northern, 

 that of 40 m. from the southern region. The Norwegian Channel with the deep 

 part of the Skager Rak may be considered a fourth hydrographical region. The 

 Skager Rak forms a funnel, through which inflows of water from both regions, 

 especially from the Norwegian Channel, enter the Belts and the Baltic through 

 the Kattegat. These inflows have here the character of under-currents, as speci- 

 fically lighter water from the Sound and the mouth of the Kattegat, forming the 

 so-called Baltic Stream, is flowing on the surface. The hydrographical profiles of 

 the Kattegat, the Belts etc., show the inflowing and outflowing water-layers in the 

 form of two wedges superimposed on one another in opposite directions (Fig. 7). 



It is only in rare cases, that the water-layers of the under- and upper-currents 

 are moving in opposite directions. Usually, the whole water-mass is moving in 

 the same direction, but in such a manner that, with a heightened hydrostatic 

 pressure from the south (caused, for example, by 

 a change of wind from the Baltic side), the upper 

 layer is moving more rapidly northwards than the 

 lower, and vice versa 2. On account of this to and 

 fro movement of the water of the under- and 

 upper-currents, the friction of the layers on one Fig. 7 



another and on the sea-bottom causes such a mix- 

 ture of waters to arise, that about two-thirds of the oceanic water which enters 

 the Kattegat is carried out again to the ocean with the Baltic Stream^. 



In the channels also, where two hydrographically different water-layers cannot English channel 

 be distinguished, as in the English Channel and the Sound (southern part), we 

 may consider the movement of the currents to be much more intermittent than 

 continuous. The inflow of water from the Channel into the North Sea ' reminds 

 one to a certain extent of the outflow of warm water (from the region of the Bay 

 of Biscay) and of cold water (from the Irish Sea) through a narrow opening. The 

 current-changes which appear at times in the Sound, have some similarity to the 

 inflow of Baltic water into Lake Malar 5. The general hydrographical situation in 

 the regions to be investigated has thus been described in the briefest manner. 



1 This relation was first discovered by the investigations of the German Gun-boat "Drache'' 

 in i8Si, 18S2 — 18S4. With regard also to the tidal currents, the Bank region, especially the Dogger 

 Bank, forms a boundary between the tidal waves entering bj' the Channel and the north of Scotland: 

 see O. Kriimmel, 1. c. 



2 Cronander, Om ytström och bottenström i Kattegat. Svenska V. A. Handl. Bd. 38, Nr. 2, 

 1894, p. 25. 



3 M. Knudsen, Ein hydrographischer Lehrsatz, Annalen der Hydrographie und maritimen Meteo- 

 rologie. Juli, 1900. 



4 Donald J. Matthews : 1. c. 



5 H. Witt and G. L,undell, Nâgra hydrographiska iakttagelser i Mälaren och Saltsjön under Fe- 

 bruar och Mars, 1895. 



