GENERAL DESCRIPTION. 297 



Ifc is apparent that circulation is more active on the Western semicircle 

 of tlie vortex tlian on the Eastern one ; and this is explained by the com- 

 bined action of various causes, as the Trade Winds, the Equatorial Currents, 

 and the Gulf Stream, also the powerful evaporation which in the Tropics 

 stimulates the circulation of the waters. 



The accompanying diagram shows graphically the drift of some of the 

 more remarkable Bottles, Derelicts, &c., recorded in the U.S. Pilot Charts. 



(245.) It may be observed that this section deals chiefly with the Surface 

 Currents of the North Atlantic Ocean, as that is the only feature which 

 affects navigation. But this superficial action is not the only point to be 

 considered in relation to Ocean Currents, as it will not explain many 

 phenomena known to exist.* The researches carried on in H.M. Ships 

 Lightning and Porcupine, in 1868. — 1870, and Challenger, 1873 — 1876, 

 under the superintendence of Dr. Carpenter, Mr. Gwyn Jeffreys, F.R.S., 

 and Professor Wyville Thomson, LL.D., F.R.S., together with the more 

 recent explorations carried out by government and private vessels of our 

 own and various other nations, have thrown much light on the temperature, 

 condition, and probable movements of the lower beds of ocean water, which 

 will be alluded to at the end of this work. Indeed, this may be considered 

 as a comparatively new branch of terrestrial physics, from which many 

 most important facts will be derived. 



(246.) That the waters of the Ocean do circulate over and intermingle 

 with every portion of the water-surface of the globe is certain. Its com- 

 position and character are everywhere, in every region, almost exactly the 

 same. This universality of character can only be accounted for by infer- 

 ring that the Ocean wat-^rs are continually being intermingled, as is the 

 case with the atmosphere, before described (2), page 98. 



It may be objected that the specific gravity of the surface water varies 

 considerably in diifereni regions, and that it is therefore an argument 

 against this intermingling of the sea-waters. But it will be found that 

 there are local causes which affect the saltness of the surface water. In 

 the Arctic regions, where it is frequently found of great density, or in- 

 creased saltness, it is doubtless caused by the formation of Ice, subtract- 

 ing the fresh water from the surface. Again, in the Equatorial regions, it 

 is usually found of low specific gravity, or containing less salt, which may 

 also be accounted for by the great rain-fall which, by intermingling the 

 light fresh water with the surface, lowers its density. Very much specula- 

 tion has been used on this variation in the surface density and on its 

 dynamic effects, in producing currents and other phenomena. 



But it is deferentially urged against this reasoning, that almost all the 

 experiments made upon the density of the water at any considerable depth 

 (above 20 or 30 fathoms) show a remarkable uniformity in the density in 

 all regions, varying within very narrow limits, from 1-024 to 1-028 at a 

 temperature of 60° F., as will be shown in a later part of this book ; and 

 that, therefore, the real character of sea-water, heloiu local influences, is 

 everywhere nearly the same. 



But we have remarkable proofs that not only the upper stratum, but also 

 the whole Ocean to its bed is of one universal character. In recent deep- 



N. A. 0. 39 



