TRANSACTIONS OF SECTION E. 701 



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 ' Blake,' the vessel specially fitted for the purpose, has dur- 

 ing 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 two miles ; a feat which 

 would a short time ago have been deemed impossible. 



One great point tbat has come out very strongly is the continual variation in 

 the strength and direction of the currents, and the varying depths to which the 

 surface current extend. 



Eastward of the chain of the Windward Islands the general depth of the sur- 

 face movement may be said to be about 100 fathoms, below which tidal influence 

 is very distinct. 



There is also a very plain backward flow of water, at depths which varj"-, 

 caused by the submarine ridge which connects the Windward Chain of the West 

 Indian Islands. These observations also generally support what I have already 

 mentioned : that the velocity of a current depends on the strength of winds, pos- 

 sibly thousands of miles distant, which have given tlie original impetus to the 

 water, and this, combined with tidal action when the current approaches or runs 

 along a coast, will always cause uncertainty on the resultant velocity. 



Dealing for yet another moment with the Gulf Stream, there are two points, 

 which have not been much dwelt upon, but which have a great effect on its power 

 of bringing the modifying influence of its warm water as far as our shores. 



The first is the prevention of its spreading, as it leaves the Strait of Florida, 

 by the pressure of the portion of the equatorial current which, unable to got 

 through the passages between the Windward Islands, is diverted to the north of 

 the Bahamas, and bears down on the eastward side of the Gulf Stream proper,, 

 compressing it between itself and the cold water flowing southward along the 

 American coast, and at the same time adding to its forces and maintaining its 

 high temperature. 



The second is that by the time the Gulf Stream has lost its velocity as a 

 current, in about the vicinity of the Banks of Newfoundland, it has arrived in the 

 region of the westerly winds, that is of winds whose average direction is from 

 west; whose influence, causing a surface drift somewhat comjparable to that of the 

 trade winds, bears the water onward to the British Islands and Norway. Without 

 these prevailing westerly winds the warm water of the Gulf Stream would never 

 reach these shores. 



The depth to which the surface currents extend in other parts is little known. 

 Direct observations on under-currents have been rare. 



In the first place, it is not an easy observation to make. Apparatus has gene- 

 rally to be improvised. This has usually consisted of some form of flat surface 

 lowered to the required depth, and suspended in the water by a buoy, which 

 presents to the resistance of the upper stratum a very much smaller area than that 

 of the surface below. 



More perfect machines have been devised, notably, that used by the Americana 

 in their West Indian experiments. 



These, however, are delicate, and require so much care and experience in work- 

 ing, and so much time is wanted for such observations, that under the pressure of 

 the more urgent requirements on surface movements in the interests of navigation 

 very little has been done. 



The ' Challenger ' made some observations on the depth of the equatorial 

 current in mid-Atlantic, but they were not very conclusive for lack of suitable 

 appliances. They, however, tended to show that below 100 fathoms there was but 

 little current. 



It has been calculated theoretically that winds blowing steadily in one direction 

 with the ordinary force of the trade winds would in 100,000 years by friction 

 between the particles put the whole of a mass of water 2,000 fathoms deep, not 

 otherwise influenced, into motion in that direction ; but the direction and force of 

 the trade winds are ever changing, and the actual strong currents of the ocean are 



