Page:Physical Geography of the Sea and its Meteorology.djvu/251

Rh born tide of the ocean. By Fig. 1 the south Pacific also outweighs the north in specific gravity; but here again the true difference, whatever it be, is somewhat masked by the time of year when the observations were made. Those north were made during the fall, winter, and spring; those south, during the fall and first winter months of that hemisphere. Nevertheless, the weight of the observations presented on Plate X. does, as far as they go, indicate that the seas of the southern do outweigh in specific gravity the seas of the northern hemisphere in the proportion of 1.0272 to 1.0262 of specific gravity. Daubeny, Dove, et al., have pointed out an excess of salt contained in sea water south of the equator, as compared with that contained in sea water north.

447. Testimony of the hydrometer in favour of the air crossings at the calm belts.—These indications, as far as they go, and this view of the subject, whatever future investigations may show to be its true worth, seem to lean in support of the idea advanced and maintained by facts and arguments in Chapter IV., viz., that the southern seas are the boiler and the northern hemisphere the condenser for the grand atmospherical engine, which sucks up vapour from the south to feed the northern hemisphere with rains. If it be true,—and Dove also thinks it is—that the clouds which supply our fountains with rain for the great American lakes, and with rains for the majestic water-courses of Europe and Asia, Northern Africa and America, are replenished from seas beyond the equator, then the waters of the ocean south should be a little Salter, and therefore specifically a little heavier, parallel for parallel, and temperature for temperature, than the waters of cis-equatorial seas. We begin to find that the hydrometer is bearing testimony in support of the evidence adduced in Chapters IV. 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 little 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 prevents currents 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