378 



NATURE 



[August ib, 1894 



mean directions of the permanent winds. It dispelled the last 

 doubt I held on the subject, as not only were the main currents 

 reproduced, but the smaller effects and peculiarities of the 

 Atlantic drifts were produced with surprising accuracy. 



There is a small current, long shown on our charts, but 

 which I had always regarded with suspicion. I refer to the 

 stream which, after travelling from the .Arctic Ocean southward 

 along the east coast of Greenland, turns sharply round Cape 

 Farewell to the northward into Davis Straits, where it again 

 doubles sharply on itself to the southward. This is exhibited, 

 in the model, in all its details, and is evidently caused by the 

 pressure of the water forced by the mimic Gulf Stream into 

 the Arctic region, where it has no escape except by this route, 

 and is pressed against the land, round which it turns as soon 

 as it can. This is, no doubt, the explanation of the real 

 current. 



The very remarkable winter equatorial current, which runs 

 in a narrow belt eastwards, just north of the main stream 

 travelling west, was also reproduced with extraordinary fidelity. 

 The winds, however, that are ordinarily considered perma- 

 nent vary greatly, while in the monsoon areas the reversal of 

 the currents caused by the opposite winds exercise a great 

 influence on the movements of the water far beyond their own 

 limits, and anything like a prediction of the precise direction 

 and rale of an oceanic stream can never be expected. 



The main facts, however, of the great currents can be most 

 certainly and simply explained in this manner. 



The trade winds are the prime motors. They cause a surface 

 drift of no great velocity over large areas in the same ceneral 

 direction as that in which they blow. These drifts after meeting 

 and combining their forces eventually impinge on the land. 



They are diverted and concentrated and increase in speed. 

 They cither pour through passages between islands, as into the 

 Caribbean Sea, are pressed up by the land, and escape by 

 the only outlets possible — as, for example, the Strait of Florida, 

 and form a great ocean current like the Gulf Stream — or, as 

 in the case of the Agulhas current and the powerful stream 

 which runs north along the Zanzibar coast, they are simply 

 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 Pacific, where the Japan current is produced in a similar 

 manner. 



The fact that on all western shores of the great oceans 

 towards which the trade winds blow we find the 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 latitudes arc 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 winds ; in 

 others they themselves originate great movements of the water. 

 Compared to the great circulation from this source the effect 

 of difTerenccs of temperature or of specific gravity is insignifi- 

 cant, though no doubt they play their part, especially in causing 

 •low undcr-circulation, and in a greater degree the vertical 

 miiing of the lower waters. 



No drop of the ocean, even at its greatest depth, is ever for 

 one moment at rest. 



Dealing with minor points, the American officers of the 

 Coast and Goedetic Survey have found after long and 

 patient investigation that the velocity of the Gulf Stream in 

 III initial and most marked pari, the .Strait of Florida, is 

 greatly affected by the tide, varying as much as onchalf its 

 maximum rate during the twenty-four hours. 



These American invcsli^;alion8 areof greatest interest. They 



have extended over the whole area of the Caribbean .Sea and 



iu approaches, the Gulf of Mexico, and the (iulf Stream 



proper and its vicinity. In no other part of the ocean has 



n of this detailed character l)een carried out, and 



* a great light on oceanic circulation. The lilakt, 



■-■""v fitted for the purpo.sc, has during the 



»evr ;, 5,he was employed on this work anchored 



in ' Hi water, or a depth of considerably more 



than lAu iiiiics ; a feat which would a short time ago have been 



deemed impossible. 



NO. 1294, VOL. 50] 



One great point that has come out very strongly is the con- 

 tinual 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 surface movement may be said to be about lOO 

 fathoms, below which tidal influence is verj' distinct. 



There is also a very plain backward flow of water, at 

 depths which vary, 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, possibly thousands of miles distant, which 

 have given the original impetus to the water, and this, com- 

 bined 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 spre.iding, as it leaves the 

 Strait of Florida, by the pressure of the portion of the 

 equatorial current which, unable to get through the pass.ages 

 betweeen 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 velo- 

 city as a current, in about the vicinity of the Bank of Newfound- 

 land, 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 comparable to that of the 

 trade winds, bears the water onward to the British Islands ani 

 Norway. Without tnese 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-current* 

 have been rare. 



In the first place, it is not an easy obser\'ation to make. 

 Apparatus has generally to be improvised. This has usually 

 consisted of some form of flat surface lowered to the require* 

 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 .\meticans in their West Indian experiments. 



These, however, are delicate, and require so much care and" 

 experience in working, and so much time is w.inted 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 curient in mid-.\tlantic, but Ihey were not very 

 conclusive for Lack of suitable appliances. They, however, 

 tended to show that below lOO 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 particle* 

 put the whole of a mass of water 2000 fathoms deep, not other- 

 wise 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 arc not in the trade wind areas, 

 but arc the result of these drifts meeting one another and being 

 compressed by the conformation of the land. We cannot,, 

 therefore, expect this theoretical effect to be realised. 



One instance of the underrunning of one current by another 

 is brought very plainly to our notice in the North Atlantic, 

 to the east of the fireat Banks of Newfoundland, where the 

 icebergs borne by ihe Arctic current from Baffin Hay pursue 

 their course to the southward across the Gulf Stream running 

 eastward. 



These great masses of ice, floating with seven-eighths of their 

 volume under the surface, draw so much water thai they are all 

 but wholly influenced by the undercurrent. A large berg will 

 have ils bollom as much as six or seven hundred feet below the 

 surface. The only reason that these bergs continue their 

 journey southward is the action of the cold under current. 



