226 rnYsiOAL geography of the sea, and its meteorology. 



land ? It is taking out of one scale of the balance and putting 

 into the other ; and the difference of specific gravity between the 

 sea water of the opposite hemisphere may give us a measure for 

 determining the amount of fresh water that is always in transitu. 

 Certainly, if evaporation and rains were to cease, if the rivers were 

 to dry up, and the sea-shells to perish, the waters of the ocean 

 would, in the course of time, become all of the same saltness, and 

 the onl}^ 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 opi^osite hemispheres, would express the ratio in volumes 

 of fresh water, whether as vapour or liquid, that would then be 

 kept in transitu between the two hemispheres. But it evaporates 

 on both sides and precipitates on both ; nevertheless, more on one 

 side than on the other, and the difference of saltness will still in- 

 dicate the proportion in transitu. If we follow the thermal and 

 specific gravity curves from the parallels of 30"^ — 34° to the equa- 

 tor {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 supposition that the 

 effects of the thermal dilatation on the sj)ecific gravity is counter- 

 acted 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 

 vapour is sucked up and caiTied 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 eva- 

 porated in one hemisphere and transported b}^ the clouds and the 

 winds to be precipitated in the other. These are questions which 

 are raised for contemplation merely; they cannot 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 alread}^ beginning to make concerning the wonders 

 of the sea. Returning from this excursion towards 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 hemisphere 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 



