[5] PRODUCTION OF OYSTERS IN A NATURAL BASIN. = 1041 
vapor, which rests over the lower portion of the mountain base, becomes 
very oppressive and sudorific, under a temperature which during my 
stay in Skyggen sometimes rose to 27° R. (92.759 F.). Ata height of a 
little over 150 feet above the surface of the lake, this strong and oppres- 
sive heat diminished more rapidly than it had done during the ascent, 
notwithstanding the fact that one had not reached the edge of the 
depression which is found in the mountain wall which surrounds the 
lake upon the north and east, and through which the newly constructed 
road leads down to North Fjord. 
3. The cooling off of the surface water produced by the radiation in 
clear nights counteracts in part the formation of the fog which origi- 
nates by the shifting of the layer of vapor resting over the mountain 
base, when this is cooled after sunset. This is, however, the case only 
jn perfectly calm weather; for when a brisk wind blew over the mount- 
ain surrounding the lake, I did not observe the formation of any fog. 
Thus, radiation deprived the water of far less heat than it had received 
during the day through the influence of the sun’s rays. 
There is no doubt that the black color of the bottom of the lake is the 
chief cause of the high temperature of the water; for when the sun’s 
rays through the crystal clear water meet the black bottom, this absorbs 
the light rays, whereby the heat rays combined with these are set free 
in the same way as in the air, and every one has felt sensibly enough 
how strong this can be when he wears black clothing on a clear, sun- 
shiny day. The heat given off from the sun-illumined bottom diffuses 
itself in accordance with physical law quickly upward to the overlying 
water stratum and communicates its temperature to the whole mass of 
water. When a body of water 26 feet deep and 2 hectares in area is 
heated up to 20° R. (77° I’.) it certainly takes a long time before this will 
cool to 10° R. (54.5° F.), especially when half the water mass—the lower- 
most—is at perfect rest, which is the case here; for the current which is 
caused by the issue of the brook into the lake certainly does -not reach 
very deep, all the more as this water is fresh and lighter than salt water, 
and consequently remains at the surface. 
In the deepest part of the lake—12 meters, or 40 feet—the saltness of 
the water is greater than in the North Sea at a depth of 4 feet. On the 
17th of October, 1878, Candidate Buch examined the saltness of the 
water in depths from 2 to 27 feet. In the latter the saltness was 3.90 
per cent., and in the former 0.20. 
In sinking the collectors at different depths they have been convinced 
that the depositing of the young does not take place below a depth of 
18 or 19 feet, and that the size and number of the young diminish in 
from 14 to 18 or 19 feet. As to the cause of this I will not venture to 
express an opinion, but the fact is fully established. 
The stratum of water in which oysters thrive best is between 3 and 14 
or 15 feet, and I have called this stratum the oyster belt of Oyster Lake. 
In this belt the swarms of young appear to congregate in at least nine 
S. Mis. 29-66 
