76 



SCIENCE. 



[Vol. VII., No. 155 



by which time the deflecting force due to the earth's 

 rotation — always to the right in the northern hemi- 

 sphere, and the contrary in the southern — has over- 

 come the eastward motion, and it now begins to 

 assume a westerly component of motion. Hence, 

 where there is an interchanging motion between the 

 equator and the poles, the effect of the earth's rota- 

 tion is torsionary, tending to give rise to an eastward 

 motion in the higher latitudes, and a westerly one in 

 the lower latitudes ; extending, where there are no 

 interruptions from continents, all around the globe. 

 The relation between these must be such that the 

 action of the former, by means of friction on the sea- 

 bottom, shall not have any greater tendency to turn 

 the earth eastward on its axis than that of the latter 

 to turn it the other way: for no change in the 

 velocity of the earth's rotation can arise from the 

 action of forces simply in the plane of the meridian, 

 which are the only real forces here, those arising 

 from the earth's rotation being simply modifying in- 

 fluences. Since the action by means of friction upon 

 the sea-bottom in the higher latitudes, which tends to 

 turn the earth from west to east, is much nearer the 

 axis of rotation than that in the lower latitudes, 

 which tends to turn it the other way, the eastward 

 motion in the former is more rapid than the west- 

 ward one in the latter. 



In the real case of nature, in which a continuous 

 motion either east or west all around the globe is in- 

 terfered with by the continents, the tendency to such 

 motions gives rise to various deflections by the conti- 

 nents. For instance: in the North Atlantic the ten- 

 dency to flow eastward in the middle and higher 

 latitudes causes a slight heaping-up of the water, and 

 a rise of surface level adjacent to the coast of Europe, 

 and a drawing-away of the water and a depression 

 of sea-level along the north-east coast of the United 

 States. As the water of the upper strata, however, 

 is thus pressed over against the coast of Europe, 

 its surface does not assume a gradient of static equi- 

 librium ; for the water, in consequence of the raising 

 of the sea-level on the coast of Europe, and especially 

 of France, is disposed of in three ways : one part is 

 deflected around to the left along the coast of Nor- 

 way, around by Spitzbergen and the east coast of 

 Greenland ; another to the right, down by the Canary 

 and Cape Verde islands in the region adjacent to the 

 north-west coast of Africa; and a small part flows 

 back westward under the upper strata as their water 

 is forced eastward. The latter is small on account 

 of the great pressure and friction on the sea-bottom, 

 which does not have its counterpart in the upper 

 strata. 



It is important to inquire here what amount of 

 motion of the water of the upper strata toward the 

 pole, arising from difference of temperature between 

 the equator and the pole, is required to cause, by 

 means of the deflecting force of the earth's rotation, 

 the necessary pressure toward the coast of Europe, 

 and raising of sea-level adjacent to it, to account for 

 the observed difference of sea-level between Brest 

 and Marseilles, and the observed resulting currents. 

 The gradient of the ocean's surface corresponding to 

 any given velocity of the water in any direction, in 

 the case of static equilibrium, may be obtained from 

 the following little table, in which the gradients are 

 given in feet per 100 miles, for a velocity of one 

 mile in twenty-four hours, the ascending gradient in 

 the northern hemisphere being always at right angles 

 to the right of the direction of motion : — 



Latitude. 



Gradient. 





Latitude. 



Gradient. 





Feet. 





Feet. 



0° 



0.000 





50° 



0.101 



10 



.023 





60 



.114 



20 



.045 





70 



.123 



30 



.066 





80 



.129 



40 



.085 





90 



.131 



From this table, it is seen that a velocity of four 

 miles per day of the water of the upper strata toward 

 the pole, on the latitude of 45°, would cause a gradient 

 of about 0.36 of a foot in 100 miles, or about 10 feet 

 between New York and Brest, in case of a static 

 equilibrium. But of course, for reasons already 

 given, there would not be really this difference, — 

 perhaps only about half of it ; but this would be 

 sufficient to account for the observed differences of 

 sea-level between Brest and Marseilles, and the Gulf 

 of Mexico and New York harbor ; the surface of the 

 ocean adjacent to the coast of France being about 

 25 feel above mean level, and that adjacent to New 

 York as much below. The velocity above, of 4 miles 

 in 24 hours, would give a very gentle and almost 

 imperceptible current, and would not be at all greater 

 than, as we have reason to think, it is. 



We have, then, an ascending gradient from the 

 north-east coast of the United States across to the 

 coast of Europe, over which the water of the upper 

 strata is impelled, until it arrives on the east side of 

 the Atlantic, by the deflecting force arising from the 

 earth's rotation and the poleward motion of the 

 water of the upper strata. From the raised sea level 

 here there is down-grade on the one hand, around by 

 the north-west coast of Africa, across the Atlantic in 

 the lower latitudes to the Caribbean Sea and Gulf of 

 Mexico, and thence to the low surface-level on the 

 west side of the Atlantic : and, on the other hand, 

 around along the coast of Norway, and by Spitz- 

 bergen and the east coast of Greenland, to the same 

 region of depressed sea-level ; both tending to fill up 

 the partial vacuum, as it is being continually main- 

 tained by the drawing-away of the waters, as ex- 

 plained above. The general descending gradient 

 from the equator toward the pole, due to a difference 

 of temperature, tends to decrease the gradient from 

 the coast of France down by the north-west coast of 

 Africa, and consequently the strength of the current ; 

 but the same increases the gradient and the strength 

 of the current on the opposite side from the Caribbean 

 Sea and the Gulf of Mexico. Hence the latter is 

 greater than the former. 



As a wide and gently flowing river, when it is con- 

 tracted into a narrow pass, becomes a rapid stream, 

 so the flow of the warm water from the Caribbean 

 Sea and the Gulf to the region of depressed ocean sur- 

 face adjacent to New York, being forced to pass 

 mostly through the Strait of Florida, becomes, in- 

 stead of a wide area of very gentle flow, as it would 

 be if it were not for the West India Islands, and 

 especially Cuba, a comparatively very narrow and 

 rapid stream, ' a river in the midst of the ocean.' As 

 this river of warm water flows northward, it tends, 

 by the effect of the earth's rotation toward the right, 

 and as the current from the east coast of Greenland 

 flows southward it is likewise deflected to the right, 

 toward the American coast. Hence, having very 

 different temperatures, and being deflected to con- 



