340 THE PHYSICAL CONDITION OF THE OCEAN. 



These observ;ition.s also generally support what I have already men- 

 tioned: 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, 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 beeu 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 iireventiou of its spread- 

 ing, as it leaves the Strait of Florida, by the i^ressure of the x^ortion of 

 the equatorial current which, unable to get through the passages 

 between the Windward Islands, is diverted to the north of the Baha- 

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

 compressing it between itself and the cold water flowing south- 

 ward 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 Bank 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 comparable 

 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 lit- 

 tle known. Direct observations on undercurrents have been rare. In 

 the first x)lace, it is not an easy observation to make. Apparatus has 

 generally to be im^jrovised. 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 ui)per stratum 

 a very much smaller area tlian that of the surface beloAv. ]\Iore perfect 

 machines have been devised, notably that used by the Americans in 

 their West Indian experiments. These, however, are delicate and 

 require so much care and experience in working, and so much time is 

 wanted for such observations, tbat under the pressure of the more 

 urgent requirements on surface movements in the interests of naviga- 

 tion 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. Tliej", however, 

 tended to show that below 100 fathoms there Avas 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 deej), 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 not in the 



