54 Currents of the Ocean. 



tropic, causes a change in the distribution of solar heat over 

 the surface of our globe, by transferring the zone of maximum 

 heat from one hemisphere to the other. As a necessary con- 

 sequence, the volume, rate, and direction of the different currents 

 are found to vary with the seasons ; cold currents preponder- 

 ating at one time of the year, and warm currents at another. 

 At the same time the areas of calms and of high or low 

 barometric pressure expand and contract, and are, to a limited 

 extent, displaced. 



Surface and Under-Currents. — It may be taken for 

 granted that, water being a ponderable substance, and, as such, 

 subject to the laws of gravity, the different strata of the ocean 

 will be found arranged according to their weight, the heavier 

 strata below, the lighter strata above. The weight of salt 

 water varies with two conditions : temperature and percentage 

 of salt held in solution. In the tropical belt, the water of the 

 surface-stratum contains more salt, the increase being due to the 

 evaporation caused by the rays of the sun. In the polar 

 regions, the quantity of salt falls below the average on account 

 of the greater proportion of fresh water derived from the 

 melting of the ice and from precipitation. 



The observations made on board the " Gazelle " have 

 shown that there is a direct relation between the colour of sea 

 water and the percentage of salt which it contains. The more 

 salt it holds in solution the more intensely blue is its colour ; the 

 less salt it contains, the more greenish the colour is. In extra- 

 tropical latitudes, we sometimes observe water of a beautiful 

 blue colour, as, for example, in the Mediterranean and other 

 nearly land-locked basins, where, the inflow of fresher water 

 being more or less cut off, the percentage of salt is raised above 

 the average by evaporation. We also observe it when crossing 

 a current coming from tropical latitudes, such as the Gulf 

 Stream. A green colour is sometimes met with in the tropics, 



