THE OCEAK. 



which it covers is quite analogous to that which rises above its 

 surface, being similarly varied by hill and valley, mountain and 

 plain. The greatest depth of the ocean is still undiscovered. 

 The plumb-line, in one part of the Pacific Ocean, was let down to 

 the depth of 27600 feet by Sir James Ross without finding bottom. 

 It has been generally assumed, as most probable, that the greatest 

 depth of the ocean does not differ much from the greatest elevation 

 of the land above its surface, which being in round numbers five 



V. THE SPANISH PENINSULA. 



miles, would show that the extreme difference of level of the solid 

 surface of the globe does not exceed ten miles, or about the 800th 

 part of its diameter. 



199. Uses of the ocean. The vast collection of water forming 

 the ocean ministers in an infinite variety of ways to the main- 

 tenance of the organised world, and in none more so than in its 

 property of evaporation. It may be considered in a certain sense 

 as a vast apparatus of distillation, by which fresh water is 

 supplied in regulated quantity and suitable quality to ail parts of 

 tne land, and in these phenomena the mountains play a con- 

 spicuous part. 



200. General system of evaporation and condensation. It 

 is demonstrated in physics that when an aqueous solution is 

 exposed to the atmosphere, the pure water which forms the chief 



198 



