352 THE PHYSICAL GEOGRAPHY OF THE SEA. 



Salter. That the waters of trans-eqiiatorial seas are Salter had 

 been pointed out by Daubeney, Dove, et al. But with a view of 

 determining by actual measurement a value for the difference 

 of saltness, I procured from Captain John Kodgers, of the North 

 Pacific Surveying Expedition, a series of hydrometrical and ther- 

 mal observations, for every degree of latitude, on a voyage in the 

 U. S. S. Yincennes, from 71° N. down through Behring's Strait 

 around Cape Horn in 57° S., and thence through the Atlantic 

 Ocean up to New York. 



These observations show that the mean specific gravity of sea 

 water is about .0007 greater in the southern than in the northern 

 hemisphere. 



The hypothesis which requires a crossing of the winds at the 

 calm belts is consistent with this fact. Half the quantity of fresh 

 water that it would take to dilute the brine of southern oceans so 

 as to reduce the specific gravity of their waters to the average of 

 cis-equatorial seas, suggests the amount of fresh water which the 

 winds of the southern hemisphere take up as vapor, carry away, 

 and do not rain down again on that side of the equator. The 

 water which is thus transported in clouds and rained upon north- 

 em fields has to find its way back to the seas of the south through 

 the currents of the ocean. Hence the difference in saltness sug- 

 gests the amount of fresh water which is perpetually in transitu 

 between the two hemispheres, as vapor through the clouds from 

 the southern, and as rain through the drainage of the land and 

 currents of the sea from the northern. This half difference would, 

 to be exact, require a farther correction on account of the inequali- 

 ty in the distribution of land and water in the "two hemispheres. 



The vapor which gives excess of precipitation to the northern 

 hemisphere is supplied from the soxithern, and it can be conveyed 

 through no other channel but the air, nor brought by any other 

 carriers but the winds. . If any portion of the air which the south- 

 east trades pour into the belt of equatorial calms passes, after ris- 

 ing up, over into the northern hemisphere, it is axiomatic that a 

 portion of like volume of that which the northeast trades pour 

 into the same belt should pass over into the southern hemisphere. 

 What may be the kind or the character of the agents to guide 

 these crossings, and lead the air from one hemisphere to the oth- 

 er, it may not be easy to discover ; it may be magnetism ; it may 



