130 THE OCEAN 



driven against a lee shore is banked up and 

 tends to descend (see Fig. 6). 



When precipitation is abundant as in 

 equatorial regions the fresh water at the sur- 

 face tends to retard vertical circulation. On 

 the other hand when winds blow from colder 

 to warmer regions, or when dry desert winds 

 blow across water, as in the case of the Red 

 Sea and Mediterranean, they are capable of 

 taking up much water-vapour from the sur- 

 face layers, which thus become more saline, 

 consequently denser, and tend to sink through 

 the subjacent layers. 



In these ways water descends and ascends 

 in the ocean, and in the descent the atmos- 

 pheric gases absorbed at the surface are car- 

 ried down to great depths, and changes in the 

 temperature and salinity are brought about. 



The foregoing remarks may be illustrated by 

 reference to the accompanying map showing 

 the surface density in the ocean (Plate VI.), 

 from which it will be noticed that the density 

 is greatest in high latitudes both of the north- 

 ern and southern hemispheres, due principally 

 to the low temperature, but where there is a 

 mixture with salt water from the tropical 

 areas the density is more pronounced than 

 elsewhere. In tropical regions there is a 

 broad continuous band around the globe in 

 which the density is less than 1-024, enclosing 



