1872.] 



' Shearwater 9 Scientific Researches. 



561 



that could only be attributed to slight differences of Temperature ; and Dr. 

 Mobius*, who has given much attention to this inquiry, has ascertained 

 that a difference of half a degree of Reaumur is quite sufficient to produce 

 sensible movement. — Further, Mr. Rainey, who has paid great attention to 

 the internal motions of small collections of liquid entirely enclosed within 

 glass and placed under the Microscope, has recently published a series of 

 observations, which show that definite currents, made obvious by the transla« 

 tion of suspended particles, take place in liquids thus enclosed, under condi- 

 tions that seem to exclude any other agencies than inappreciable differences 

 of Temperature. (St. Thomas's Hospital Reports, New Series, vols. i. & ii.) 



37. I have not, indeed, learned that any eminent Physicist endorses Mr. 

 CrolPs objections to the doctrine I advocate ; whilst I have, on the other 

 hand, had the satisfaction of receiving from both Sir John Herschel 

 and Sir William Thomson an explicit acceptance of it. The former was 

 kind enough to write to me, only a short time before his last illness, as 

 follows : — " Assuredly, after well considering all you say, as well as the com- 

 " mon sense of the matter, and the experience of our hot water circulation- 

 " pipes in our greenhouses, &c, there is no refusing to admit that an Oceanic 

 " circulation of some sort must arise from mere Heat, Cold, and Evaporation, 

 " as verce causae ; and you have brought forward with singular emphasis the 

 " more powerful action of Polar Cold, — or rather, the more intense action, 

 " as its maximum effect is limited to a much smaller area than that of the 

 " maximum of Equatorial Heat. The action of the Trade and Counter-trade 

 " Winds in like manner cannot be ignored ; and henceforward the question 

 " of Ocean-currents will have to be considered under a twofold point of view. 

 " The Wind-currents, however, are of easier investigation. All the causes lie 

 " on the surface : none of the agencies escape our notice ; the configuration 

 " of coasts, which mainly determines their direction, is patent to sight. It is 

 " otherwise with the other class of movements : they take place in the depths 

 " of the Ocean ; and their movements, and directions, and channels of concen- 

 " tration, are limited by the configuration of the sea-bottom, which has to be 

 " studied over its whole extent by the very imperfect method of Sounding. " 



38. Imperfect as that method is, however, I cannot but believe that a 

 sufficient number of Serial Temperature-soundings, taken in different parts 

 of the Oceanic area, especially in the Southern Hemisphere, will afford 

 adequate data for the settlement of the question whether such a Circulation 

 actually exists ; and also that means may be found for determining its rate 

 by observation and experiment. If such determinations can be made, I 

 am assured by Mathematicians that they will afford valuable data for the 

 accurate determination of the " viscosity " of water, which is at present by 

 no means the known quantity assumed by Mr. Croll. It is consequently 

 with great satisfaction that I look forward to the results of the inquiries to 

 be prosecuted in this direction by the Circumnavigation Expedition now 

 being fitted out by Her Majesty's Government. 



* See 1 Annals of Natural History,' Ser. 4, vol. viii. p. 201. 



