Mar. 14, 1872J 



NATURE 



385 



dicular to it, as he did the former, he would have 

 found that the parts in the direction of motion, arising 

 from both components, exactly cancel one another in all 

 cases, and that the resultant of both components is 

 a force perpendicular to the direction of motion. This 

 force then tends only to change the direction of the mo- 

 tion, and never to accelerate or retard it, in whatever 

 direction it may be. Prof Colding's result, therefore, that 

 the velocity of the current is accelerated by the earth's 

 rotation, when moving in certain directions, and retarded 

 in others, is erroneous. 

 j It is known that there are two theories with regard to 



I the cause of Ocean Currents : the one, that they are 

 caused by the winds acting upon the ocean, the other, ad- 

 vocated by Dr. Carpenter, that they are caused by a dif- 

 ference of density of the ocean between the equator and 

 the poles, due to a difference of temperature. The ten- 

 dency of both theories is in the same direction, and the 

 currents, no doubt, are in some measure due to the forces 

 belonging to each theory. The history of the former 

 theory, and the high authority which cjn be appealed 

 to in its support, are well known, but we have reason 

 to think that the forces, and the effects of them, in the 

 former theory, are quite subordinate to those of the 

 latter. The well-known explanation of the Gulf Stream 

 by the former theory assumes that there is a heaping 

 up of the water of the ocean in the Gulf of Mexico by 

 the action of the trade winds, sufficient to change the 

 sea-level enough to cause the observed current passing 

 through the Strait of Florida. But the trade winds 

 cannot have much effect in causing a heaping up of 

 the water on the coast of Mexico, since the force is 

 applied to the surface merely, and tends to produce 

 only a surface current, while all the great body of the 

 water, except a little of the surface, is free to flow back. 

 It is true there must be a slight change of sea-level to 

 give rise to a force sufficient to overcome the resistances to 

 this under tow, but these are extremely small since the 

 velocity of this under tow, including all the great depth 

 of the ocean, except the superficial westward current, is 

 very small. That the merely superficial part of the equa- 

 " torial current is mostly caused by the trade-winds may be 

 true, but the Gulf Stream, which is not directly acted 

 upon, except by the very gentle south-west winds, and 

 which is not merely a surface-current, must be mostly ac- 

 counted for by the other theory. Let us now see what can 

 be learned upon this subject from observation. Instances 

 of a great change of water-level in shallow canals have 

 been cited to show the influence of the wind in causing a 

 heaping up of the water at the one end ; but the water in 

 these cases being very shallow, the force may be regarded 

 as applied somewhat to the whole body of the water, and 

 the under counter-current is thus prevented, but the case 

 is very different in a deep ocean. It is well known from 

 the discussion of tidal observations that the influence of 

 the wind in changing the sea-level is very small. If the 

 force of the trade winds causes a higher sea-level in the 

 Gulf of Mexico, we know that the west winds in higher 

 latitudes must cause a similar rise of sea-level on the west 

 coast of Europe, for the sum of the moments with refer- 

 ence to the earth's axis, of the forces, v/est between the 

 tropics and east in higher latitudes, must exactly balance 

 each other. If the explanation of the Gulf Stream requires 

 that the level of the Gulf of Mexico should be raised about 

 twelve feet, as shown by Prof Colding, then there must be 

 about an equal change of level on the west coast of Europe, 

 if these changes are caused by the winds ; for although 

 the extent of coast receiving the west winds may be greater 

 than that receiving the east winds, yet this is counter- 

 balanced by the circumstance that the force of the west 

 winds acts at a less distance from the earth's axis, which 

 requires that they should be stronger. If, then, the west 

 winds cause a change of sea-level on the coast of Europe, 

 say of ten feet, then any change in the force of these 



winds at different seasons must cause a very perceptible 

 change of sea-level. Now, we know that the force of the 

 west wind on the Atlantic Ocean is considerably greater 

 in the spring than the autumn. There should therefore be a 

 corresponding difference in the mean level of the sea, and 

 this mean level on the coast of Europe should be greatest 

 in the spring. But the discussion of the tidal observations 

 made at Brest, shows that the mean level of the sea, after 

 being corrected for the barometer and a very small astro- 

 nomical term affecting the mean level, is about four inches 

 lower in the spring when the winds are strongest than in 

 the autumn when they are weakest. (Proceedings of the 

 American Academy of Sciences and Arts, vol. vii. p. 32.) 

 The discussion, likewise, of the tides of Boston Harbour 

 gives a similar result, except that the range of the monthly 

 means is still less, being less than three inches. (U.S. 

 Coast Survey Report for 1868.) These results should re- 

 ceive the attention of those who maintain that great 

 changes of sea-level are caused by the winds. 



In a paper by the writer, published in Silliinan's 

 youriial (second series, vol. xxxi. p. 45) there are several 

 pages given to the subject of ocean currents, in which it 

 is maintained that the principal agency in their production 

 is difference of temperature of the sea-water between the 

 equator and the poles. The principal effects of the earth's 

 rotation are there given, which are too numerous to be re- 

 cited here. In addition to the results there given, the 

 following additional thought may be given here as being 

 perhaps new. As the surface-water flows toward the poles 

 the deflecting force of the earth's rotation presses it toward 

 the east. In like manner as the water below flows toward 

 the equator, there is a similar force pressing it toward the 

 west. These forces are small, but they must nevertheless 

 cause a gradual rising of the cold water at the bottom on 

 the American coast, and this, perhaps more than the Green- 

 land current, causes cold water there. The Gulf Stream 

 of warmer water cuts its way through this cold water 

 gradually rising from the bottom, and hence the cold walls 

 observed by the U.S. Coast Survey. 



Mr. Croll seems committed to the wind theor)', and is 

 unwilling to admit that the theory advocated by Dr. 

 Carpenter can have even a subordinate effect. His princi- 

 pal argument is based upon an experiment of M. Dubuat. 

 I know not under what circumstances this experiment was 

 made, but of course it was with a comparatively shallow 

 canal or stratum of water, and the result is no doubt 

 correct for the depth of water with which the experiment 

 was made. A much less force on each particle of a large 

 body of water is sufficient to overcome the cohesion of the 

 particles, and produce motion than upon a small one, just 

 as a small drop of water remains suspended to a twig, 

 while the same force of gravity causes a large one to drop 

 off. The case therefore of the ocean is very different from 

 that of a shallow canal. As Mr. Croll insists that Dr. 

 Carpenter's experiment, to be applicable to the case, should 

 have been made with a canal 120 feet long, and only one 

 inch deep ; so it might be insisted that M. Dubuat's ex- 

 periment, to be applicable to Mr. CroU's case, should be 

 made with a canal or body of water three or four miles 

 deep. But there is no necessity for us to make any such 

 experiments, for nature is performing the experiment 

 regularly every six hours, and all that we have to do is to 

 observe. The attraction of the moon changes the level 

 due to the attraction of the earth alone, and puts the 

 ocean, as it were, upon an inclined plane with a gradient 

 of about two feet in the distance of a quadrant, and the 

 water slides down, causing a rising of the tide at one 

 place and a falling at another ; and in six hours this 

 gradient is reversed, and the reverse motion of the water 

 follows, thus causing the regular ebbing and flowing of 

 the tides. If M. Dubuat's experiment were applicable to 

 the ocean, the moon could not cause a tide at all unless 

 its mass were about fifteen times greater. 



Cambridge, Mass., U.S.A. W. Ferrel 



