SEA WATER 223 



and in general there is a decrease in temperature with depth. 

 Usually the effects of the sun's warmth are not felt to a 

 greater depth than three hundred fathoms Actually 

 the heat rays are rapidly absorbed by the upper few inches 

 of water and any warmth that is transported to deeper 

 layers is mostly brought about by the mixing of the warmer 

 with the colder water. 



Owing to this very slow warming up of the sea there is 

 a lag in the seasonal change of temperature, and while 

 in our latitudes the hottest time of the year is June, the 

 water does not reach its maximum temperature until 

 August. Now we have seen that the deeper layers only 

 receive their heat by mixing with the warm upper water 

 and while there is a lag between the raising of the 

 temperature of the air and that of the water there is a 

 further lag in the warming of the deeper water, until at 

 about fifty fathoms we get a complete reversal of seasons. 

 There the hottest part of the year is in December, that is 

 mid-winter, and the coldest about May or June. Below a 

 hundred fathoms there is no seasonal change whatever in 

 temperature, and, year in year out, the conditions are 

 uniform. From this depth downwards the temperature 

 gradually falls until in the ocean abysses it remains always 

 at somewhere near the freezing point. 



The actual weight or specific gravity of the water depends 

 on its temperature and upon its salinity, the warm water 

 being lighter than the cool, and the fresh water lighter 

 than salt. In the tropics therefore the high temperature 

 makes the water light, but at the same time owing to 

 evaporation the salinity is high, which tends to make the 

 water heavy. 



The pressure in the sea varies with the depth. At every 

 ten metres depth the pressure is increased by one atmos- 

 phere, that is by 14 lbs. to the square inch. In the great 



