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 scientifie subjects :— 
“CoLLINGwoop, 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 
ereenhouses, &c., there 1s no refusing to admit that an oceanic 
circulation of some sort must arise from mere heat, cold, and 
evaporation, as vere cause, 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. 
“JT 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 some accidental substitution of one 
bottle for another, or from evaporation. I never put any hearty 
faith in it. 
