378 OBSERVATIONS ON THE CURRENTS. 



such a slope (of 1-lOth of a second, 0-1" of arc), is less than one irwo- 

 millionth part of gravity, we may dismiss this as a cause capable of creat- 

 ing only a very trifling surface drift, not worth considering, even were it 

 the proper direction to form, by concentration, a current from East to 

 West ; which it woidd not he, hut the very reverse.'^* 



With regard to this, Lieutenant Pillsbury, U.S.N., states that the tem- 

 perature of the Ocean in the vicinity of the Equator reaches 37° F., at a 

 depth of about 1,000 fathoms, below which the changes are slight to the 

 bottom. The surface has n temperature not far from 80°. The expansion 

 of the water due to this change in temperature is about -005, which for the 

 column of 1,000 fathoms would be about 30 feet, giving a grade from the 

 Equator to the Poles of about -06 inch to a mile, an infinitesimal difference. 



Professor Perrel considers that the effect of a difference of Temperature 

 in the water of the Ocean between the Equatorial and Polar regions is 

 similar to its effect on the atmosphere. The initial effect, before motion 

 ensues, is to raise the surface of the Ocean at the Equator above that of 

 the Polar regions (calculated by Mr. Croll to be 4^ feet), thus giving rise 

 to a motion poleward, compensated by an under-flow from the Poles to the 

 Equator. He also denies that the Winds are the origin of the Currents, 

 and holds that they are primarily due to differences of Temperature, acted 

 on by the effect of the earth's rotation. f 



We may here draw atte(*tion to an ingenious theory advanced by General 

 H. Mathiesen, of the Danish Army, founded on the study of special sm^ace 

 temperature charts. He combats the generally accepted idea of the circu- 

 lation of the waters of the Ocean, holding that the surface isothermal lines 

 (about which more will be said at the end of this work) indicate warm 

 cm-rents from the Equator towards the Poles, and cold currents from the 

 Poles towards the Equator, the former being of a clear blue colour, and the 

 latter discoloured by various organic substances. He argues that the Winds 

 cannot be considered as the cause of the permanent Oceanic Currents.]: 



Evaporation has been assumed as another cause, but it can be shown 

 that the lines of maximum evaporation are near the Tropics, that is, near 

 to the point from which the Gulf Stream removes and flows away, instead 

 of rurming towards, if this be taken as a sole cause. But the line of greatest 

 precipitation is near the Equator, and therefore the surface water of the 

 Ocean is there lighter, or of less specific gravity, than under the evapo- 

 rating Tropical influences ; this source of instability will cause the waters 

 to flow directly towards this line, throughout the whole circuit, and not in 

 the form of a partial stream. Besides this, it is shown by the experiments 

 of deep-sea exploring vessels, that the density of sea- water, at some fathoms 

 below the surface, is very nearly the same all over the ocean (246), so that 

 surface experiments afford but imperfect data, upon which little or nothing 

 can be grounded in the present state of our knowledge. 



(363.) There have been so many objections raised to the plain fact that 



• " Physical Geography," by S^r John F. W. Herschel, 1861. 



t "Recent Advances in Meteorology," by William Ferrel, M.A., Ph.D., WashingtoB, 



U.S., 1886. 



, :♦ " E'tude sur les Courants et sur la Temperature des Eaux de la Mer dans rOo^an 

 'Atlantique," par le G^n^ral H. Mathiesen ; Christiania, 1892. 



