378 THE DEPTHS OF THE SEA. [chap. viii. 



and excellent letter addressed to Dr. Carpenter — a 

 letter which there is no impropriety in my quoting in 

 full as it is already in print, and which has a special 

 interest as being probably one of the last written 

 by Sir John Herschel on scientific subjects : — 



" Collingwood, April 9th, 1871. 



"My dear Sir, — Many thanks for your paper on the Gib- 

 raltar current and the Gulf-stream. Assuredly, after well con- 

 sidering all you say, as well as the common 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 vera causce, and you have brought forward with 

 singular emphasis the more powerful action of the 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 studied 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 concentra- 

 tion are limited to the configuration of the sea-bottom, which 

 has to be studied over its entire surface by the very imperfect 

 method of sounding. 



" I am glad you succeeded in getting specimens of Mediter- 

 ranean water near the place of the presumed salt spring of 

 Smyth and Wollaston, making it clear that the whole affair 

 must have arisen from sume accidental substitution of one 

 bottle for another, or from evaporation. I never put any hearty 

 faith in it. 



