180 THE PHYSICAL GEOGRAPHY OF THE SEA. 



water is generally ; but this circumstance is due to local causes 

 of easy explanation. For instance : when we come to an arm of 

 the sea, as the Red Sea (§ 404), upon which it never rains, and 

 from which the atmosphere is continually abstracting, by evapor- 

 ation, fresh water from the salt, we may naturally expect to find 

 a greater proportion of salt in the sea water that remains than we 

 do near the mouth of some great river, as tlie Amazon, or in the 

 regions of constant precipitation, or other parts where it rains 

 more than it evaporates. Therefore we do not find sea water 

 from all parts of the ocean actually of the same degree of salt- 

 ness, yet we do find, as in the case of the Red Sea, sea water that 

 is continually giving off to evaporation fresh water in large quan- 

 tities ; nevertheless, for such water there is a degree, and a very 

 moderate degree, of saltness which is a maximum ; and we more- 

 over find that, though the constituents of sea water, like those of 

 the atmosphere, are not for eveiy place invariably the same as to 

 their proportions, yet they are the same, or nearly the same, as to 

 their character. 



493. "When, therefore, we take into consideration the fact that, 

 as a general rule, sea water is, with the exceptions above stated, 

 every where and always the same, and that it can only be made 

 so by being well shaken together, we find grounds on which to 

 base the conjecture that the ocean has its system of circulation, 

 which is probably as complete and not less wonderful than is the 

 circulation of blood through the human system. 



494. In order to investigate the currents of the sea, and to 

 catch a glimpse of the laws by which the circulation of its waters 

 is governed, hypothesis, in the present meagre state of absolute 

 knowledge with regard to the subject, seems to be as necessary to 

 progress as is a corner-stone to a building. To make progress 

 with such investigations, we want something to build upon. In 

 the absence of facts, we are sometimes permitted to suppose them; 

 only, in supposing them, we should take not only the possible, 

 but the probable ; and in making the selection of the various hy- 

 potheses which are suggested, we are bound to prefer that one by 

 which the greatest number of phenomena can be reconciled. 

 When we have found, tried, and offered such an one, we are en- 



