Page:Physical Geography of the Sea and its Meteorology.djvu/252

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 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 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 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 supposition that the effects of the thermal dilatation on the specific 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 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 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 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 already 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