466 



THE POPULAR SCIENCE MONTHLY. 



depths of the ocean there is no trace of snnlight. It is highly im- 

 probable, on the face of it, that any ray of light could penetrate 

 through a stratum of water four miles in thickness, even if the 

 water were perfectly pure and clear, but when we remember that 

 the upper regions, at least, are crowded with pelagic organisms 

 provided with skeletons of lime and silica, we may justly consider 

 that it is impossible. 



The temperature of the water in the abyss is by no means con- 

 stant for a constant depth, nor does it vary with the latitude. It 



is true that, as a rule, the 

 water is colder at greater 

 depths than in shallower 

 ones, and that the deeper 

 the thermometer is low- 

 ered into the sea, the low- 

 er the mercury sinks. 

 This is consistent with 

 physical laws. If there 

 is any difference at all in 

 the temperature of a col- 

 umn of water that has 

 had time to settle, the 

 thermometer will always 

 reach its highest point at 

 the top of the column and 

 its lowest at the bottom, 

 for the colder particles 

 being of greater specific 

 gravity than the warmer ones will sink, and the warmer ones will 

 rise. The truth of this will be clear if we imagine a locality at 

 the bottom of a deep ocean with a source of great heat such as an 

 active volcano. 



Such a source of heat would, it is true, raise the temperature 

 of the water in its immediate vicinity, but the particles of water 

 thus heated would immediately commence to rise through the 

 superjacent layers of colder water, and colder particles would fall 

 to take their places. Thus the effect of an active volcano at the 

 bottom of the deep sea would not be apparent at any very great 

 distance in the same plane. In fact, unless the bottom of the ocean 

 was closely studded with volcanoes we should expect to find, as in- 

 deed we do find, that the temperature of the sea rises as the water 

 shallows. 



If then we were to consider a great ocean as simply a huge 

 basin of water, we should expect to find the water at the surface 

 warmer than the water at the bottom. The temperature of the 

 surface would vary constantly with the temperature of the air 



Fig. 2. SicYONis ckassa : M, mouth ; S, ciliated 

 groove ; T, tentacles. Each tentacle is perforated 

 by a single large aperture. (After Ilertwig.) 



