RECENT HYDROBIOLOGICAL INVESTIGATIONS 487 



sea become cooled by radiation they become denser and sink 

 to the lower levels, setting up vertical convection currents 

 which are of the greatest importance with regard to the 

 distribution of the marine plankton. In the spring of the 

 year the air is warmed, and for a short period there is thermic 

 equilibrium between the sea and the atmosphere. Then the 

 atmosphere, becomes the warmer, and heat is communicated 

 from it to the surface layers of the sea, with the result that 

 there is usually, at this season, a warmer surface stratum 

 overlying colder and denser water. The temperature at the 

 surface of the sea is not therefore a good measure of the 

 relative amount of heat conveyed by the oceanic drifts, except 

 in the case of coastal waters of less than about one hundred 

 fathoms in depth. There we usually find a strong mixture 

 of the water, which is due to tidal streams, and the whole 

 body of water may be practically homogeneous as regards 

 temperature and salinity. The best measure of the varying 

 influence of the Gulf Stream drift is the amount of heat 

 contained in that part of the water which is still unmixed 

 with other water of different origin. 



The observations under review thus prove — what has indeed 

 long been known in a general kind of way — that the climates 

 of Norway and Sweden, and to a less extent that of the British 

 Isles, are much milder than would be the case if the Gulf 

 Stream did not reach those shores. Really the observations 

 do more than this, for they indicate the probability that the 

 character of the seasons in the Scandinavian countries may 

 be predicted about six mouths to a year ahead; while that of the 

 sea off the Murman coast might be foretold about two years in 

 advance. The water which fills the Iceland-Faeroe Channel 

 flows on and fills the sea off the coasts of Northern Norway 

 about a year and a half later, and the basin of the Barentz 

 Sea about two and a half years later. If, then, we know the 

 exact physical conditions of the water at such a " strategic 

 point" as the Faeroe Channel, from month to month, we might 

 be able to foretell the character of the seasons in the Northern 

 countries some considerable time in advance. This is the 

 conclusion of great practical importance which emerges from 

 the results of the hydrographic study of the seas of Northern 

 Europe. It does not seem that we shall ever be able to 

 predict the exact nature of what we term the "weather" from 



