DISCOVDUY 



375 



Examining first the question of currents by con- 

 vection, we observe : 



(i) The heat of the sun is appHed from above ; the 

 surface water is therefore subject to most heat. As 

 heated water is expanded and is therefore lightened, 

 there will, from this cause, be no tendency for the 

 surface water to change places with the lower water, 

 as would be the case if the heat were applied from 

 below. But, as against this, is the fact that ocean 

 water contains a fair proportion of salt, which gives a 

 Specific Gravity of, roughly, vozj, i.e. for every 

 1,000 ozs. of pure water which it contains there are 

 27 ozs. of salt. The effect of heat in the Tropics on 

 the salt water will be to evaporate the pure water, 

 lea\ing the residuum with greater saltness. From 

 this cause the surface water will become the heavier 

 and, consequently, will sink. Whatever is the re- 

 sultant in these two opposite cases, the motion will be 

 vertical, and as such would bear little relation to 

 the great horizontal currents which we are considering. 



(2) There is, next, the condition of the unequal 

 heating of the surface waters of the globe, due to the 

 respective positions relative to the Equator and the 

 Poles. Regarded as the primitive and sole cause for 

 the great ocean currents, the components contained 

 in these conditions are insufficient to account for the 

 necessary displacements. Gravity can have no direct 

 effect in drawing polar water to the Equator, and 

 equatorial water to the Poles. And here also there 

 arise as in {1) two complementary and opposite sets 

 of conditions ; viz. {a) that cold, pure water is heavier 

 than warm, pure water ; and {b) that much of the 

 cold water of the Arctic oceans will be pure owing to 

 the melting of glaciers and icebergs which, being 

 frozen pure water, will, when melted, be of less specific 

 gravity than the salt water of the oceans. This 

 latter, (6), may, however, be of trifling consideration 

 when the vast ocean waters are being considered. If 

 the Arctic oceans arc heavier than the equatorial, 

 then the action would naturally be of a creeping or 

 sliding — i.e. indirect — nature, the warmer water from 

 the Equator remaining on the surface. But this 

 creeping in folds in broad stretches from north to 

 south and south to north, is not in accordance with 

 the definite cyclic movement which takes place in the 

 ocean, and therefore this influence of itself fails to 

 account for the phenomena. Regarded, however, as 

 a secondary influence in the operation of currents, the 

 idea of differences of specific gravity from temperature 

 may be considerable. For instance, if a cold current 

 (caused by other influences) meets a warm current, there 

 may at once be displacements in which the colder water 

 will sink and the warmer water rise. This seems to be 

 the case with the Gulf Stream and Labrador currents. 



(3) Considering next the question of prevailing winds 



as causes of the ocean currents, we find here a 

 considerable correspondence between the two, and, 

 because wind can influence the surface flow of water, 

 it has been inferred that the winds are the causes of 

 the ocean currents. This, however, does not neces- 

 sarily follow ; the very fact that these two fluids, the 

 atmosphere of the earth and the water of the earth, 

 have corresponding motions, may simply indicate that 

 they are susceptible to the same cause or causes. The 

 more freely moving air may then, by friction, accentuate 

 the slower, tardier, movement of the ocean. There is, 

 however, an example in which the action of the wind, 

 among the factors which operate in the causation of 

 currents, seems to be predominant. Against the 

 normal flow we find that the north equatorial current 

 of the Indian Ocean is changed in its direction from 

 west to east under the influence of the Summer Mon- 

 soon. It is probable, however, that all wind action 

 is very superficial. 



The fundamental action on which all currents depend 

 for their prime initiation can without hesitation, 

 because of the strong evidences which arise to support 

 it, be ascribed to the rotation of the earth. Questions 

 of temperature, the action of winds, etc.; sink into 

 insignificance compared with the essential cause which 

 is found in rotation. These are but modifying factors 

 of the main factor. The rotation of the earth is the 

 backbone of oceanic circulation. 



The rotation of the earth represents a movement 

 round a fixed axis. In this movement, the solid earth, 

 the waters of the ocean and the gases of the atmosphere, 

 all take part. But, whereas the solid crust of the earth 

 is rigid and a constant relation of its parts to one 

 another is kept up, on the other hand, the atmosphere 

 and the oceans being fluid, movements of considerable 

 magnitude are constantly going on within themselves, 

 while as a whole they rotate. But also, in the actual 

 rotation of the solid parts and the fluid parts of the 

 earth, there is not a strict relation and fixity of action. 

 Consider what happens if a tank of water is pushed 

 violently awa\". The tank moves — the whole of it ; 

 the water, however, slops over towards the side pushed. 

 Relatively to the tank only, the water moves in 

 a direction opposite to the tank. Two main factors 

 are accountable for this. First, there is the inertia 

 of the water which gives it a quality or tendency to 

 remain where it was — the tank also had inertia. 

 Secondly, it possesses the quality of fluidity wherewitli, 

 partlv, its inertia can escape the effects of the push. 

 In other words, owing to its lack of rigidity the push 

 is not conveyed " end on " throughout the particles 

 of the liquid. The condition of the earth is similar 

 to this. In its rotation the waters of the ocean do not 

 keep pace with the solid crust ; they lag behind. 

 WTiile, therefore, the rotation is from west to east. 



