228 THE SEAS 



fathoms in thickness, is sufficient to support the tremendous 

 wealth of plant life that forms the pasturage of the sea, 

 and it is on the rain of dead plants and organisms that 

 have fed on them that the animals in the dark ocean depths 

 depend for their food. It is hard for us to realize what the 

 actual strength of the light is at different depths in the 

 sea. Not enough accurate work has as yet been done for 

 us to be able to make actual comparisons ; but it has 

 recently been shown at Plymouth that in the open waters 

 of the English Channel the light at a depth of about sixty 

 feet corresponds to that found in the heart of an English 

 beech wood, and this is quite dim. At such a depth in 

 the Mediterranean however the light would be very much 

 stronger, not only because the sun itself is so much more 

 powerful, but because the water is clear and lacks that 

 sediment of sand and mud that so discolours the water 

 in the English Channel. 



Currents 



The great oceans of the world are not merely masses of 

 stationary water. There exists within them a system of 

 gigantic currents whereby the water is continually cir- 

 culating and moving from place to place. 



Amongst these currents is that known as the " Gulf 

 Stream," a name familiar to all on account of its bearing 

 on the climate of our islands. In order to understand 

 roughly how the Gulf Stream takes its being and how it 

 moves, it is necessary to consider the whole of the North 

 Atlantic ocean. The Gulf Stream is a great oceanic current 

 that receives its name from its place of origin, the Gulf 

 of Mexico, whence it issues through the Straits of Florida 

 as a stupendous river of warm blue water fifty miles in 

 width and three hundred and fifty fathoms deep. 



The forces that set and keep this great mass of water in 



