502 



NATURE 



[April 2<„ 1872 



yournal of Science and Arts, a description of a fish-nest which 

 Prof. Agassiz obtained from the seaweed of the Sargasso Sea in 

 December last. 



In this interesting paper Prof. Agassiz identifies the embryos 

 which he acquired from the nest as the young of the Cheiromctes 

 fictus, which, as its name implies, has fins like hands. From the 

 description given I have no doubt but that my specimen is the 

 Cheit'oncctes, and I lose no time in forwarding to you the result of 

 my reading. J. E. Me[;yon 



H.M.S. Duke of Wellington, Portsmouth 



OCEAN CURRENTS 

 TN the Philosophical Magazine for October 1870 and 

 ■•■ 1 87 1 I have examined at considerable length the 

 arguments which have been advanced in favour of the 

 theory that Oceanic Circulation is due to differences of 

 specific gravity between the ocean in equatorial and polar 

 regions. Since then a point in reference to the infl uence 

 ot the earth's rotation has suggested itself to my mind 

 ■which appears to be wholly irreconcilable with the gravi- 

 tation theory of currents. 



It is one of the properties of a fluid that the resistance 

 which it offers to motion is equal in all directions. It 

 follows, therefore, that when an ocean current is flowing 

 in any particular direction, the forces acting on the moving 

 water must be greatest in the direction of motion. 

 According to the theory that oceanic circulation is due to 

 difference of specific gravity, resulting from the difference 

 of temperature between the equatorial and polar waters, 

 the direction of motion at the surface of the ocean is from 

 the equator to the poles, and at the bottom from the poles 

 to the equator, subject to a deflection caused by the 

 earth's rotation. According to this theory gravity tends 

 to impel the water from the equator towards the poles 

 along the line of meridian ; while rotation tends to deflect 

 the water towards the cast. If the total amount of work 

 performed on the moving water by these two forces were 

 equal, then the water on the northern hemisphere would 

 take a north-easterly direction, and that on the 

 southern hemisphere a south-easterly direction. But 

 owing to the way in which the two forces vary in relation 

 to each other, the path taken is not a straight line but a 

 curve, the particular character of which has been deter- 

 mined with great labour by Mr. Ferrel. 



But whatever view we may adopt in regard to the in- 

 fluence of rotation on the moving waters, whether it be 

 that advocated by Dr. Colding and others, or that pro- 

 pounded by Mr. Ferrel, it is evident that if we assume the 

 amount of the impelling energy of gravity to be not 

 greater than the deflecting energy of rotation, we shall be 

 led to the conclusions that there can be no such general 

 interchange of equatorial and polar water in the Atlantic 

 as Dr. Carpenter maintains. For under such conditions 

 water leaving the equatorial regions for the Arctic seas 

 would move as rapidly eastward as northward, and would 

 consequently be deflected against the western coast of the 

 old continent, and arrested in its progress before it reached 

 even the latitude of England. 



I need not, however, dwell further on this point, for I 

 do not suppose there are any advocates of the gravitation 

 theory who will not freely admit that the impelling energy 

 is at least equal to the deflecting energy, and if this be 

 admitted, it is all that is necessary for my present argu- 

 ment. 



What proportion then does the impelling energy of 

 gravity bear to the deflecting energy of rotation ? 



The velocity of rotation at the equator is about 1,526 

 feet per second, and at lat. 60'', about 773 feet per 

 second. Were water frictionless, and did it offer no 

 resistance to motion, then a pound of water flowing 

 from the equator in the direction of the pole would, 

 on arriving at latitude 60", have, according to hitherto re- 

 ceived ideas, an easterly velocity relative to the earth's 

 surface of 763 feet per second. Mr. Ferrel has, however, 



shown that the relative velocity would be much greater. 

 But not to run the risk of over-estim.ating the velocity, I 

 shall be content to take it at 763 feet. Water flowing 

 from the equator towards the poles, instead of having an 

 actual velocity of 763 feet per second on reaching latitude 

 60°, has, at the utmost, a velocity not over one or two feet. 

 If we suppose the velocity to be, say, 3 feet per second, 

 then 760 feet per second of velocity derived from rotation 

 is consumed by friction and other resistances in the pas- 

 sage of the water from the equator to that place. A 

 pound of water moving with a velocity of 760 feet per 

 second possesses in virtueof that velocity°9,025 foot-pounds 

 of energy. This enormous amount of energy is all con- 

 sumed, not in impelling the pound of water from the 

 equator to latitude 60", but in simply deflecting it to the 

 east during its motion. Consequently 9,025 foot-pounds 

 is the amount of energy required to perform the work of 

 deflection. But since the resistance offered by a fluid to 

 motion is equal in all directions, the resistance offered to 

 the impelling force must be as great as that offered to the 

 deflecting force. It is, I trust, admitted that in the pas- 

 sage of the pound of water from the equator to latitude 

 60"', the distance traversed by the water under the influence 

 of the impelling force is as great as the distance traversed 

 under the influence of the deflecting force, or, in other 

 words, the distance from the equator to latitude 6p°, 

 measured along the meridian, is as great as the distance 

 to which the water is deflected to the east during its pas- 

 sage. Then, if this be the case, 9,025 foot-pounds of 

 energy of the impelling force must be also consumed in 

 overcoming the resistance to the motion of the pound of 

 water ; that is, the impelling force requires to perform 9,025 

 foot-pounds of work before it can convey a pound of water 

 from the equator to latitude 60°. Can gravitation, there- 

 fore, be the impelling force ? Can gravity, according to 

 Dr. Carpenter's theory, perform 9,025 foot-pounds of work 

 on a pound of water in impelling it from the Equator to 

 latitude 60° ? 



Taking Dr. Carpenter's own data as to the temperature 

 of the ocean at the poles and equator, and the rate at 

 which the temperature at the equator decreases from the 

 surface downwards, I have shown* that 9 foot-pounds is 

 the greatest amount of work which gravity can perform 

 on a pound of water (placed under the most favourable 

 circumstances) in carrying it from the equator to either 

 pole. Assuming the slope from the equator to the poles 

 to be uniform, 6 foot-pounds will be the total amount of 

 work that gravity can perform upon a pound of water 

 in its passage from the equator to lat. 60°. But this is 

 only J jigf, part of the amount of energy required. Hence, 

 if there is any circulation of water between the equatorial 

 and polar regions, it must be produced by a cause 1,500 

 times more powerful than the one to which he appeals. 



But in reality the amount of energy impelling the 

 water must be far more than 1,500 times greater than 

 what can be derived from gravity, for the water moves 

 more in the direction of the impelling force than in the 

 direction of the deflecting force, thus proving that the im- 

 pelling force is greater than the deflecting force. 



Although it will be admitted that the resistance offered 

 by fluid friction is equal in all directions, yet it may be 

 urged that, owing to the influence of the winds or some 

 other cause or causes which I have not taken into account, 

 the actual resistance to motion may be greater in some 

 directions than others. This no doubt may be the case, 

 but it cannot possibly affect the conclusion at which I 

 have arrived, unless it be shown that the resistance to 

 pole-ward motion is 1,500 times less than the resistance 

 to eastward motion. 



But these results are as conclusive against the theories 

 of Maury, Colding, Ferrel, and in fact against every pos- 

 sible form of the gravitation theory, as against the theory 

 of Dr. Carpenter. And I need hardly add that they are 

 equally fatal to the theory that ocean currents are caused 

 * Phil. Mag., Oct. 1871, 



