HYDROGR. SECTION — APPENDIX D (52) 
a relatively high salinity, are to a very large extent mixed with one another. And 
it has been proved that this mixing differs much according to the various seasons, 
and a regular sequence in these differences has been discovered; so that we are. 
now pretty well informed with regard to the various water-masses which enter 
the Baltic during the various seasons of the year. Besides the currents, the 
salinities and temperatures have also been minutely examined, and it has been ~ 
proved that well-marked yearly periods are discernible, which may be very diffe- 
rent at different places and depths. On the regularity of these periods detailed 
information has been gained. Unperiodical variations, however, are very conspicuous, 
and these must be studied continuously, in order to elucidate their connection 
with other phenomena. 
Ill The Skager Rak: | 
In the Skager Rak also, as well as in the North Sea, a distinct yearly pe- 
riodicity underlies the hydrographie phenomena. These variations are most con- 
spicuous in the relatively fresh water of the coastal areas. Numerous facts have 
been learned about these variations, as well as about their connection with the 
presence and abundance of the various species of plankton organisms. | 
The relatively fresh surface-layer in the Skager Rak shows temperature 
variations that are large in proportion to those of the deeper layers. | 
A large water-layer, of salinity greater than 35 00, fills up the basin of the 
Skager Rak from the bottom to 60—150 m. from the surface, and consists of 
Gulfstream water, which has flowed down from the north into the North Sea and 
thence into the Skager Rak. 
IV. North Sea: 
Water of high salinity streams into the North Sea from the Atlantie Ocean, 
in part through the English Channel but mainly by the North. The salt stream 
entering by the Channel becomes mixed with the North Sea water especially by 
the strong tidal currents. It must be borne in mind, however, that the action of 
strong winds, often prevalent in the region, on the progress of Channel water and, 
its mixing with North Sea water, may be very great. Considerable changes in the 
allernating tidal currents during stormy weather, have beea pointed out by obser- 
vations on certain lightships. 
These changes, although temporary, if occurring at certain times of the year, | 
may produce great alterations in the physical conditions of life in the shallow 
and narrow parts, and greatly modify the transported surface plankton. 
