1872.] 



e Shearwater ' Scientific Researches. 



549 



17. Two separate questions have to be considered, which have not, 

 perhaps, been kept sufficiently distinct either by Mr. Croll or by myself : — 

 first, whether there is adequate evidence of the existence of a General 

 vertical Oceanic Circulation ; and second, whether, supposing its existence 

 to be provisionally admitted, a vera causa can be found for it in the dif- 

 ference of Temperature between the Oceanic waters of the Polar and the 

 Equatorial Areas. It is obvious that while the admission of the existence 

 of such a Circulation by no means involves the acceptance of the explana- 

 tion of it which I have offered, no disproof of the validity of that expla- 

 nation can neutralize the inferential evidence in its favour which is afforded 

 by the facts of observation. But it is also obvious that if a Force can be 

 shown to be in constant operation, which is adequate to produce the effect 

 I assign to it, the inferences based on the facts of observation are greatly 

 strengthened. And I cannot too strongly protest against the statement 

 that no body of observations can establish a doctrine, in opposition to an 

 asserted demonstration of its impossibility, the probative value of which 

 entirely rests upon a single set of observations, and upon the relevancy of 

 those observations to the question under discussion. 



18. I am indebted to the Presidential Address of Mr. Prestwich to the 

 Geological Society in 1871 for the following summary of the older doctrine 

 on this subject, which had been generally lost sight of in this country 

 through the prevalence of the erroneous doctrine of the universality of the 

 temperature of 39° in the Deep Sea, to which I have alluded in former 

 Reports (1868, p. 186, 1869, § 123): — " Humboldt states (Fragmens de 

 Geol. et de Climatol. Asiat., 1831) that he showed in 1812 that the low tem- 

 perature of the tropical seas at great depths could only be owing to currents 

 from the Poles to the Equator. — D'Aubuisson, in 1819, also attributed the 

 low temperature of the sea at great depths at or near the Equator to the 

 flow of currents from the Poles (Traite de Geognosie, p. 450). — Lenz, in 

 1831, gave the results of some experiments he had made at great depths in 

 the ocean ; and concluded that between the Equator and Lat. 45° the 

 temperature decreases regularly to the depth of 6000 feet, when the de- 

 crease becomes insensible. The lowest temperature he recorded was 36° 

 Fahr. (Edinb. Journ. of Science, vol. vi. p. 341). — Pouillet briefly dis- 

 cusses Ocean-temperatures, and concludes that, although all the difficulties 

 of the case are not solved, it seems certain that there is generally an upper 

 current carrying the warm Tropical waters towards the Polar seas, and an 

 under-current carrying the cold waters of the Arctic regions from the Poles 

 towards the Equator (Elem. de Phys. 1847, torn. ii. p. 667)." — The doctrine 



Roderick Murchison (speaking through Sir Henry Rawlinson) is reported to hare said, 

 with reference to the Paper I read on that occasion : — " From what he had heard of tho 

 " Paper on the Law of Oceanic Circulation, it appeared to him that if its conclusions were 

 " borne out by experiment, the announcement would rank, amongst the discoveries in 

 " Physical Geography, on a par with the discovery of tho Circulat ion of the Blood in 

 " Physiology." 



