THE PHYSICAL CONDITION OF THE OCEAN. 345 



the land, ami escape by the only outlets i^ossible — as, for example, the 

 Strait of Florida, and form a great ocean current like the Gulf Steam — 

 or, as in the case of the Agulhas current and the powerful stream which 

 runs north along the Zanzibar coast, they are simi)ly pressed up against 

 and diverted by the land, and run along it with increased rapidity. 

 These rapid currents are eventually apparently lost in the oceans, but 

 they in their turn originate movements of a slower character, which on 

 again passing over shallow water or on meeting land develop once more 

 into well-defined currents. 



We find an analogous state of things on the western side of the Pa- 

 cific, where the Japau current is produced in a similar manner. The fact 

 that on all western shores of the great oceans toward which the trade 

 winds blow we find tbe strongest currents running along the coast, is 

 almost enough of itself to prove the connection between them. 



The westerly winds that prevail in higher northern and southern lati- 

 tudes are next in order in producing great currents. From the shape 

 of the land they in some cases take up and continue the circulation 

 commenced by the trade whids; in others they themselves originate 

 great movements of the water. Compared to the great circulation from 

 this source the effect of differences of temperature or of specific grav- 

 ity is insignificant, though no doubt they play their part, especially in 

 causing slow under circulation, and in a great degree the vertical mix- 

 ing of the lower waters. No drop of the ocean, even at its greatest 

 depth, is ever for one moment at rest. 



Dealing with minor iwints, the American officers of the Coast and 

 Geodetic Survey have found after long and i)atient investigation that 

 the velocity of the Gulf Stream in its initial aiul most marked part, the 

 Strait of Florida, is greatly affected by the tide, varying as much as 

 one-half its maximum rate during the twenty-four hours. These Amer- 

 ican investigations are of greatest interest. They have extended over 

 the whole area of the Caribbean Sea and its approaches, the Gulf of 

 Mexico, and the Gulf Stream proper and its vicinity. In no other part 

 of the ocean has observation of this detailed character been carried 

 out, and they throw a great light on oceanic circulation. The BlaJce, 

 the vessel specially fitted for the purpose, has during the several years 

 in which she was employed on this work anchored in over 2,000 fathoms 

 water, or a depth of considerably more than 2 miles, a feat which would 

 a short time ago have been deemed impossible. One great point that 

 has come out very strongly is the continual variation in the strength 

 and direction of the currents and the varying depths to w^hich the sur- 

 face current extends. 



Eastward of the chain of the Windward Islands the general depth 

 of the surfiice movement may be said to be about 100 fathoms, below 

 which tidal influence is very distinct. There is also a very plain back- 

 ward flow of water, at depths which vary, caused by the snbmarine 

 ridge which connects the Windward chain of the West Indian Islands. 



