NATURE 
THURSDAY, JUNE 8, 1871 


THE GENERAL OCEANIC CIRCULATION 
MONG the results of the Porcupine Expeditions of 
1869 and 1870, there are perhaps none more impor- 
tant than those relating to the Temperature of the Deep 
Sea. For it is only to such accurate determinations of 
ocean temperatures as have now been made for the first 
time, not only at the surface and the bottom, but also at 
intermediate depths, that a really scientific theory can be 
framed of that great Oceanic Circulation, which, while it 
eludes all ordinary means of direct observation, seems to 
produce a far more important effect, both on terrestrial 
climate and on the distribution of the marine fauna, than 
that of the entire aggregate of the surface-currents which 
are more patent to sight. The latter usually have winds 
for their prime motors, and their direction is mainly deter- 
mined by the configuration of the land; so that their 
course and action will change with any superficial altera- 
tion which either opens out a new passage or blocks up 
anold one. The former, on the other hand, depending 
solely on difference of temperature, will (to use Sir J. 
Herschel’s apposite language) have its movements, direc- 
tion, and channels of concentration mainly determined 
by the configuration of the sea-bottom ; and vast eleva- 
tions and subsidences may take place in this, without pro- 
ducing any change that is discernible at the surface. 
The history of the doctrine of the general oceanic 
circulation has been recently given in the Anniversary 
Address of the President of the Geological Society, with | 
a completeness which (so far as we are aware) had never 
been previously paralleled. But this doctrine has hitherto 
rested on the very insecure foundation of observations 
which were alike inadequate and inaccurate ; and it has 
consequently been discredited, both by physicists and by 
physical geographers. It is now impossible to assign a 
precise value to the older observations upon deep-sea 
temperatures. For it was shown by the careful 
experiments which were made by Mr. Casella two years 
ago, under the direction of thelate Prof. W. A. Miller, 
Dr. Carpenter, and Captain Davis of the Admiralty, that 
the pressure of sea-water at great depths on the bulb of 
the thermometer—a pressure amounting to about a ton 
per square inch for every 800 fathoms—exerts so great 
an influence on even the very best instruments of the 
ordinary construction, as to cause a rise of eight or ten 
degrees under an amount equivalent to that which would 
be exerted at from 2,000 to 2,500 fathoms’ depth ;* and 
the error of many thermometers under the same pressure 
was two or three times that amount. There is reason to 
believe that some of the thermometers formerly employed, 
especially in the French scientific expeditions, were pro- 
tected against that influence; but no such protection 
appears to have been applied to the thermometers sup- 
plied to Sir James Ross’s Antarctic Expedition ; and the | 
observations by which he supposed himself to have estab- 
lished the existence of a uniform deep-sea temperature of 
* Mr. Prestwich cites Dr. Carpenter as estimating the error from pres- 
sure “‘at 2° or 3° or even more.” The error is said by Dr. Carpenter to have 
been from 2° to 3° on the depths of from 500 to 700 fathoms first explored ; 
_ but would have been from 8° to 10° at the depths subsequently reached. 
VOL, IV. 

oF 

about 39°, now seem to have been altogether fallacious. 
So again, Captain Spratt’s observations in the Mediter- 
ranean, though made with great care, were seriously 
vitiated by this source of error. 
It appears from Mr. Prestwich’s exhaustive summary, 
that as long ago as 1812 Humboldt had maintained that 
such a low temperature exists at great depths in tropical 
seas, as can only be accounted for by the hypothesis of 
under currents from the Poles to the Equator. And this 
view was adopted by D’Aubuisson, Lenz, and Pouillet ; 
the latter of whom considered it 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 to the Equator.” Our Arctic navigators had met 
with temperatures in the Polar seas as low as 29° at 1,000 
fathoms ; and these observations have been more recently 
confirmed by those of M. Charles Martins and others in 
the neighbourhood of Spitzbergen. Several instances are 
| recorded, on the other hand, in which temperatures of 
from 38° to 35° were observed at great depths nearly under 
the Equator ; and this alike in the Atlantic, Pacific, and 
Indian Oceans. 
The Temperature-soundings taken in the Lightaing and 
Porcupine Expeditions, with trustworthy instruments, have 
shown :—(1) That in the channel of from 600 to 700 
fathoms’ depth which lies between the North of Scotland, 
the Orkney and Shetland Islands, and the Faroes, there 
is an upper stratum of which the temperature is con- 
siderably higher than the normal of the latitude ; whilst 
there is stratum occupying the lower half of this channel, 
of which the temperature ranges as low as from 32° to 
29°5; and a “stratum of intermixture” lying between 
these two, in which the temperature rapidly falls—as much 
as 15° in 100 fathoms. (2.) That off the coast of Portugal, 
beneath the surface-stratum, which (like that of the 
Mediterranean) is super-heated during the summer by 
direct solar radiation, there is a nearly uniform tempera- 
ture down to about 800 fathoms; but that there is a 
“stratum of intermixture” about 200 fathoms thick, in 
which the thermometer sinks 9°; and that below 1,000 
fathoms the temperature ranges from 39° down to about 
36°'5. (3.) That in the Mediterranean the temperature 
beneath the super-heated surface-stratum is uniform to 
any depth ; being at 1,500 or 1,700 fathoms whatever it 
is at 100 fathoms, namely from 56° to 54°, according to 
the locality. To these may be added (4) the observations 
| recently made by Commander Chimmo, with the like 
trustworthy thermometers, which, in lat. 3° 184’ S., and 
long. 95° 39’ E., gave 35°°2 as the bottom temperature at 
1,806 fathoms and 33°6 at 2,306 fathoms. These seem 
to be the lowest temperatures yet observed in any part of 
the deep ocean basins outside the Polar area. 
It is clear, therefore, that very strong evidence now 
exists, that instead of a uniform deep-sea temperature of 
39°, which, on the authority of Sir James Ross, by whom the 
doctrine was first promulgated, and of Sir J. Herschel, by 
whom it was acceptedand fathered, had come tobe generally 
accepted in this country at the time when the recent deep- 
sea explorations commenced, not only is the temperature of 
the deeper parts of the Arctic basin below the freezing- 
point of fresh water, but the temperature of the deepest 
parts of the great oceanic basins, even under the Equator, 
G 
