THE SPECIFIC GRAVITY OF THE SEA, ETC. 217 



f 



is bearing testimony in support of the evidence adduced in Chapters 

 lY. and VII., to show that when the trade-winds meet and rise up 

 in the equatorial calm belt, the atmosphere which came there as 

 south-east trade-winds passes with its vapour over into the northern 

 hemisphere. We had not anticipated that this httle instrument 

 could throw any light upon this subject ; but if, as it indicates, the 

 sea water of the other hemisphere be Salter and heavier than the 

 sea water of this, what makes it so but evaporation, and what pre- 

 vents cm-rents from restoring its equilibrium but the winds, which 

 are continually sucking up from the brine of trans -equatorial seas 

 and pouring it down as fresh water upon cis-equatorial seas and 

 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 measm-e for 

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

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

 to dry uj), 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 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, v,'ould 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 ciu:ves 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 specific gTavity 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 measm-e for the amount of water which as 

 vapour 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 eva- 

 porated in one hemisphere and transported by the clouds and the 

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



