PHYSICAL OCEANOGRAPHY 225 



and pressure, and the geographical distribution of animals, 

 later on. 



The high temperature at the surface evidenced by the curves Absorption of 

 in Fig. 157, is principally due to the absorption of heat-rays from surtaceJ!? Ae 

 the sun. In places the water is heated by contact with warm sea. 

 air, but this source of heat is of less importance, the temperature 

 of the surface-water being, as a rule, higher than the temperature 

 of the air. The sun's rays penetrate into the water and are 

 absorbed ; the dark heat-rays are absorbed in the uppermost 

 layers, while the light rays, which also convey a little heat, 

 make their way down to a depth of several hundred metres 

 before disappearing altogether. The action of the sun's rays is 

 strongest in the tropics, declining towards the north and south, 

 and this in a general way explains the distribution of the surface- 

 temperature. 



A fine example of the heating action of the sun's rays is storage of 

 afforded by the Norwegian oyster-basins. Along the west jj^nl" ""^""^"^ 

 coast of Norway there are in many places salt-water basins, 

 separated from the outer fjord by a sill, which is covered only 

 at high water. At the surface the water of the " poll " — as 

 such a basin is called in Norway — is comparatively fresh, 

 and consequently light ; from a depth of about one metre to the 

 bottom it is very salt and heavy. The sun's rays in summer 

 penetrate into the water and heat it, mostly at the surface, but 

 also to some extent down to a depth of a few metres. The 

 surface-water is cooled during the night, but at a depth of one 

 or two metres beneath the surface the heat will not be given 

 off so readily, because the heavy water there does not reach the 

 surface. When this has gone on for some time, the temperature 

 at a depth of a few metres may be remarkably high, sometimes 

 fully 35° C, while the temperature at the surface might be 

 about 20° C. In these "polls" the surface-layer of relatively 

 fresh water prevents the layers below from coming into contact 

 with the cooling air, and such polls may indeed be compared to 

 hot-houses, the fresh surface-layer corresponding to the fixed 

 transparent roof, under which heat is stored.^ in these 

 oyster-basins absolutely tropical conditions are developed in 

 summer. It is significant that Gran once found in one of them 

 a small crustacean, which according to G. O. Sars belongs to 

 the Guinea Coast. Fig. 158 shows the temperatures and salini- 

 ties in an oyster-basin in the early part of the summer before 



1 Compare Murray and PuUar, Bathy metrical Sui~i<ey of the Fresh- Water Lochs of Scotland, 

 vol. i. pp. 580, 581, and 587, Edinburgh, 1910. 



Q 



