Oct. 21, 1875] 



NATURE 



533 



but in vegetative characters— such as shape of leaves and 

 arrangement of flowers) were dispersed in broad outline 

 as at present, before present islands were insulated and 

 the present general dispersion of sea and land worked 

 out. The reader will find in the volume a very large 

 amount of information on these subjects compressed into 

 a small space. 



LETTERS TO THE EDITOR 



[ TTic Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return^ 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications.~[ 

 Ocean Circulation 



Having carefully read Mr. Croll's papers in the Philosophical 

 Magazine for September and October, I find in them the full 

 confirmation of my statement that his " crucial-test " argument 

 is based on the assumption of an equilibrium between the Equa- 

 torial and the North Atlantic columns; the words "to be in 

 equilibrium" or " in order to equilibrium " being used over and 

 over again to fix this as the essential condition of the compu- 

 tation by which the North Atlantic column is made out to 

 be 2>\ feet higher than the Equatorial. 



No reference to other passages in Mr. Croll's writings can 

 countervail this fact. I pointed out at Bristol the fallacy it in- 

 volves, which was at once recognised by Sir William Thomson, 

 General Strachey, and other competent authorities. This fal- 

 lacy becomes obvious in the following parallel case : — 



The specific gravity of /Egean water being to that of Black 

 Sea water as (say) 1029 to 1013, a column of Black Sea 

 water 1,029 feet high would be required to balance a column of 

 i^gean water 1,013 feet high ; therefore (on Mr. Croll's assump- 

 tion of an equilibrium) the level of the Black Sea must be above 

 that of the /Egean in the proportion of 16 feet to 1,013 feet of 

 depth. But that there is not an equilibrium between the two 

 columns, is conclusively proved by the deep inflow of /Egean 

 water which always accompanies the surface-outflow of Black 

 Sea water, showing the ^gean column to be the heavfer. 



Now Mr, CroU has obviously no more right to assume an 

 equilibrium between the North Atlantic and the Equatorial 

 columns, and thereby to deduce from their relative tempera- 

 tures the higher level of the fonner, and the consequent impos- 

 .sibilily of the thermal circulation as making the poleward upper 

 flow run uphill, than he would have to deduce the excess of 

 level of the Black Sea from its lower salinity, and to assert that 

 an inward underflow of ^gean water is impossil^le, as tending 

 to raise that level yet higher. 



But there is yet another serious error in Mr. Croll's compu- 

 tation, which, even admitting his fundamental assumption, com- 

 pletely invalidates his conclusion. He has entirely omitted the 

 consideration of the inferior salinity of the Equatorial column ; 

 which, as it shows itself alike at the surface and at the bottom, 

 may be fairly taken as characterising its entire height. This 

 will make a difference in the opposite direction of about one foot 

 in 1,026; sufficient, therefore, if the excess in the North At- 

 lantic column extends to a depth of no more than 600 fathoms, 

 to neutralise the whole 3^ feet of elevation which Mr. Croll 

 deduces from relative temperatures. 



Mr. Croll is unable to see what the "viscosity" of water has 

 to do with the question. Just this — that it affects his whole doc- 

 trine of "gradients." The nearer water is to a "perfect fluid," 

 the less is the gradient required to give it horizontal motion. 



If a viscous fluid be drawn from the bottom of one end,/^, of a 

 long trough A—B, its level at i? will be lowered more slowly than 

 at A, and will remain appreciably higher so long as the outflow 

 continues. But in the case of a "perfect fluid" and a slow 

 outflow, the level will practically fall simultaneously along the 

 whole length of the trough A — B. I am quite aware that, mathe- 

 matically speaking, the level must be always lower at A than at 

 B ; since there can be no movement of any particle from B 

 towards A, unless room has been previously made for it. 

 But if the time required for the replacement of each particle by 

 the one next adjacent to it be infinitely small, the excess of re- 

 duction at A will also be infinitely small. 



Now, according to the authorities I previously cited, water 



approaches so nearly to the condition of a "perfect fluid," that 



very small differences in its density will suffice (if constantly re- 



ewed) to maintain a vertical circulation, without any appreciable 



difference in let'el. And my position is, that the void created by 

 the slow descent of water chilled by the surface-cold of the Polar 

 area will be so speedily replaced by the inflow of water from die 

 circumpolar area, and this again by inflow from the temperate 

 region, as to produce a continual upper-flow of equatorial water 

 towards the pole, without the gradient which Mr. Croll per- 

 sistently asserts to be necessary. 



I now leave it in the hands, not of Mr. Croll, but of com- 

 petent authorities in Physics, to decide (i) whether his " crucial 

 test " has the value he himself assigns to it, and (2) whether his 

 doctrine of "gradients" can stand examination by the light now 

 thrown upon it by Mr. Froude's researches. Until some physicist 

 of equal weight with Sir George Airy and Sir William Thom- 

 son shall pronounce the doctrine I advocate to be untenable, I 

 shall continue to believe, with Lenz, Arago, and Pouillet, that 

 it is the only one which can account for the phenomena of Deep- 

 sea temperature. 



That the temperature of the upper stratum of the ocean is 

 often affected by the Wind-circulation, and is especially thus 

 modified in the North Atlantic, I have repeatedly pointed out. 

 Audit is scarcely fair in Mr. Croll, therefore, to continue speaking 

 of the " wind-theory " and the "gravitation-theory" of Ocean 

 Circulation as if they were antagonistic, insteadof being not only 

 compatible, but mutually complementary — the wind-circulation 

 being horizontal, and the thermal circulation vertical. 



As, however, Mr. Croll has now advanced so far as to admit 

 that "physicists may diflfer from him in regard to whether or 

 not the present difierence of temperature between the ocean in 

 equatorial and polar regions is sufficient to produce circulation," 

 I am not without hope that in another year or two he may come 

 to accept the Thermal-circulation as a " great fact ; " and that he 

 may then make good use of his knowledge and ability in eluci- 

 dating the shares which are taken by the Wind-ciiculation and 

 the Thermal-circulation respectively, in the distribution of ter- 

 restrial heat. William B. Carpenter 



The Sliding Seat 



Most problems in animal mechanics are of so complicated a 

 character as to be generally referred to direct experiment rather 

 than to mathematical analysis. 



In Mr. Wagstaffe's remarks (vol. xii. p. 3^9) on the analogy 

 which exists between the movements at the sterno-clavicular 

 articulation in rowing, and those permitted by the sliding seat, 

 we have an argument in favour of the latter arrangement. But 

 when the subject is regarded from the point of view assumed by 

 a practical oarsman, the question of actual^ advantage still re- 

 mains unanswered. 



There are certain preliminaries which must be considered 

 before we can commence to solve the problem, leading to its 

 subdivision into several distinct problems, some of which will 

 prove interesting to the anatomist, some to the mechanician, 

 some to the physiologist In the following remarks I shall 

 attempt to indicate the preliminaries referred to. 



LFic. a. 



Fig. I represents the position of the vertebral axis, v A, the 

 thigh, TH, and the leg, H L, when the point a cr the seat is 

 fixed. 



Fig. 2 exhibits the same parts when a' is movable. In both 

 there is the same position for the outstretched arms, that is, 

 m n = m' n'. 



It is clear that in i the weight, w, will be raised by such 

 forces as tend to move v a towards the vertical position ; while 

 in 2 the same result is obtained by changing v'a' without 

 alteration of the angle of inclinatioi^ We thus see that the 

 angles a and (^ will vaiy in definite inverse ratio, while the varia- 



