Huntsman. — Climates of Our Atlantic Waters 329 



water is being cooled. Another case is the action of the tidal 

 currents which churn up the water and not only carry the sur- 

 face conditions into the deeper water, but also bring the less va- 

 riable bottom conditions to the surface, making the surface 

 water, as in the Bay of Fundy, cooler and more salt than it 

 would otherwise be. These currents change not only the tem- 

 perature of the water, but also its salinity, its gas content, and its 

 alkalinity or acidity. 



Of a very different nature are the effects of depth upon the 

 water climate. We have already referred to the absorption of 

 the sun's rays by water. In the most transparent sea the red 

 rays are nearly all stopped before a depth of 250 fathoms is 

 reached and not many rays of any kind penetrate deeper than 

 500 fathoms. The depths of the sea experience a never ending 

 starless night and the intermediate waters enjoy a brief daily 

 bluish twilight. In coastal waters invariably with more or less 

 sediment, sunlight penetrates the water to a very much slighter 

 depth. 



The gain and the loss of heat occurs at the surface of the 

 water, and as the latter is a poor conductor, the circle of the 

 seasons is felt only in its upper layers except when vertical cur- 

 rents come into play. These currents are chiefly caused by the 

 density of the surface water becoming greater than that below. 

 This occurs in autumn when cooling is in progress, the water 

 becoming denser as its temperature falls. The situation is com- 

 plicated in fresh water by the fact that below 40° F. water 

 becomes less instead of more dense as it gets colder, and in salt 

 water by the fact that the density of the water depends not only 

 upon the temperature but also upon the salinity. 



Water gains and loses at its surface not only heat but also 

 many of the gases it contains. They diffuse but slowly through 

 its substance and, therefore, only its upper layers, except where 

 vertical currents interfere, will have a rather constant content 

 of these gases in equilibrium with their condition in the air. 

 Elsewhere the local production and consumption of the gases 

 by organisms will dominate their abundance. 



