1903-4.] Prof. C. G. Knott on Ocean Temperatures , etc. 183 
temperature of the maximum density even of fresh water, and 
consequently the same type of vertical temperature distribution 
remains permanent throughout the year. In the Mediterranean 
we are therefore dealing with a permanent average distribution of 
temperature which is the steady resultant effect of ages of solar 
radiation, convective cooling, and heat conduction, down from the 
warmer surface waters and up from the slightly warmer earth 
below the cold bottom waters. 
Superposed upon this steady average distribution we have the 
daily see-saw of temperature due to direct solar radiation and to 
the complex indirect effects which accompany it. As the sun rises 
the surface waters become heated, and to some extent evaporate. 
This may cause increased salinity in the surface waters, and give 
rise to gravitation convection currents. Ripples, waves, migration 
of fish aid the mixing of the waters, so that down to a depth 
of perhaps 5 or 10 metres the temperature distribution is largely 
affected by these causes, the pure conduction effect being compara- 
tively unimportant. The direct heating effect of solar radiation at 
depths greater than 15 metres may be regarded as negligible, 
because of the great absorption of solar energy in the water near 
the surface. From the Pola records we know that luminosity 
can penetrate to considerable depths, for white objects at depths of 
50 metres were frequently visible. But these rays must be robbed 
of by far the greater part of their original energy, which, indeed, 
has gone to heat the surface waters. As evening comes on 
evaporation will largely cease, the surface waters will cool off by 
radiation, and convection will be set up which will last well 
through the night, warmer water continually welling up to replace 
the cooler heavier water which sinks. By this means the tempera- 
ture throughout the upper layers becomes steadily reduced, and 
the heat gained in the day is lost at night. During the day the 
process of heating is mainly due to the radiant energy of the sun 
being absorbed by the water near the surface, aided by mechanical 
mixing of the layers of water. At night the process of convection 
tends to bring to the surface all the water comprised within a 
layer whose depth will depend upon the temperature reached 
during the day, the rate of cooling of the surface during the 
night, and the viscosity of the water. The depth to which 
