224: THE PHYSICAL GEOGRAPHY OF THE SEA. 



waters of tlie ocean would, in the course of time, become all of the 

 same saltness, and the only difference of specific gravity in the sea 

 would be due to thermal agencies. After having thus ceased, if 

 evaporation were then to commence only in the other hemisphere, 

 and condensation take place only in this, half the difference, as to 

 saltness of the sea water in opposite hemispheres, would express 

 the ratio in volumes of fresh water, whether as vapor or liquid, 

 that would then be kept in transitu between the two hemispheres. 

 But it evaporates on both sides and precipitates on both ; never- 

 theless, more on one side than on the other, and the difference of 

 saltness will still indicate the proportion in transitu. If we follow 

 the thermal and specific gravity curves from the parallels of 30° — 

 34° to the equator {Figs. 1 and 2, Plate X.), we see, as I have said, 

 that sea water in this part of the ocean does not grow lighter in 

 proportion as it grows warmer. This is accounted for on the sup- 

 position (§ 435) that the efiects of the thermal dilatation on the spe- 

 cific gravity is counteracted by evaporation. Now, if we knew 

 the thickness of the stratum which supplies the fresh water for this 

 evaporation, we should not only have a measure for the amount 

 of water which as vapor is sucked up and carried off from the 

 trade-wind regions of the sea, to be deposited in showers on other 

 parts of the earth, but we should be enabled to determine also the 

 quantity which is evaporated in one hemisphere and transported 

 by the clouds and the winds to be precipitated in the other. 

 These are questions which are raised for contemplation merely ; 

 they can not be answered now ; they grow out of some of the 

 many grand and imposing thoughts suggested by the study of the 

 revelations which the hydrometer is already beginning to make 

 concerning the wonders of the sea. Keturning from this excur- 

 sion toward the fields of speculation, it will be perceived that these 

 observations upon the temperature and density of sea water have 

 for their object to weigh the seas, and to measure in the opposite 

 scales of a balance the specific gravity of the waters of one hemi- 

 sphere with the specific gravity of the waters of the other. This 

 problem is quite within the compass of this exquisite system of 

 research to solve. But, in order to weigh the seas in this manner, 

 it is necessary that the little hydrometric balance by which it is 

 to be done should be well and truly adjusted. 



448. From these premises it would not be difiicult to show that 



