April 6, 1871| 

Whilst we were prosecuting these inquiries, our attention was 
attracted by the long chains of aggregate salpze which were float- 
ing close to the ship near the surface of the very calm sea. We 
were able to collect four or five different species of these, and to 
submit them during life to microscopic examination. The re- 
versal of the direction of the circulation took place in all at more 
regular intervals than we have usually found to be the case in the 
compound ascidians ; and we were able to distinguish an un- 
mistakable rudimentary eye, which had not, we believe, been 
previously noticed. We hope to be able hereafter, by the 
detailed study of these specimens, to make some additions 
to the knowledge previously acquired of this very interest- 
ing group. As the nature of the bottom put it out of the 
question to attempt to dredge on this ridge, our only means 
of investigating its zoology lay in the use of the ‘““hempen 
tangles.” A ‘‘ sweep” taken with these brought up a few 
echinoderms and polyzoa of no special interest. 
We now took our final leave of the Mediterranean basin with 
mingled feelings of disappointment and satisfaction. The zoolo- 
gical results of our cruise had been by no means equal to our 
expectations ; but, on the other hand, we could console ourselves 
with the belief that our determination of the peculiar physical 
conditions of this great inland sea, and in particular our eluci- 
dation of the mystery of the Gibraltar current, would be fairly 
regarded as a success. And we venture to think that this will 
be admitted by such as may follow us through the discussion of 
general results, to which we shall presently proceed. 
As Captain Calver considered himself bound not to make any 
unnecessary delay in returning homewards, and to take every 
advantage of the continuance of the fair weather and favourable 
breeze which we enjoyed during nearly the whole remainder of 
our voyage, we were reluctantly compelled to give up the idea of 
prosecuting any further researches in the Deep Sea ; and devoted 
ourselves to the examination of the specimens previously col- 
lected, and to the correlation of our temperature and other results, | 
—specially directing our attention, however, to the surface- 
temperature of the emdouchure of the Strait, with the view of 
ascertaining whether a sudden /a// would be observable on quit- 
ting it, corresponding to the 77se which had been noticed on the 
outward voyage on entering it. 
very decided. As we kept along the southern coast of Portugal 
NATURE 

This change proved to be | 
towards (ape St. Vincent, the surface-temperature averaged — 
73°°5- 
At © P.M. we were turning the corner of the Cape, 
and found the surface-tempeiature 72°°5. 
And at 8 p.M., 
when we were fairly in the Atlantic, we found that the surface- | 
temperature had fallen to 69°, thus showing a difference of 4°°5. 
On the following day, when we were off Lisbon, the surface- 
temperature was 69°°5 ; and it gradually diminished as we pro- 
ceeded northwards from that point. Although the season of the 
year led us to expect a rough passage across the Bay of Biscay, 
the weather continued remarkably fine until we reached the 
**Chops of the Channel,” where we fell in with rather a fresh 
breeze ; this did not interfere, however, with our anchoring at 
Cowes on the afternoon of the next day (October 8th), after an 
absence of just two months, during which a greater number of 
most important public events had occurred than had ever before 
been crowded within so short a period. 
General Oceanic Circulation.—The difference as to level and 
density between two bodies of sea-water, which produces the ver- 
tical circulation in the Strait of Gibraltar and the Baltic Sound, 
may be brought about otherwise than by the excess of evapora- 
tion which maintains it in the one case, or by the continual 
dilution with fresh water which maintains it in the other. It 
may be easily shown that a constant and decided difference of 
temperature must have exactly the same effect. Let the Medi- 
terranean basin be supposed to be filled with water of the same 
density as that of the Atlantic and up to the same level ; and to 
be then cooled down below the freezing-point of fresh water by 
the withdrawal of solar heat, whilst the surface of the Atlantic 
continues to be heated as at present by the almost tropical sun- 
shine of the Gibraltar summer. The cooling of the Mediterranean 
column, reducing its bulk without any diminution of weight, 
would at the same time lower its level and increase its density. 
An in-draught of Atlantic water must take place through the 
Strait to restore that level ; but this in-draught would augment 
the weight of the column, giving it an excess above that of the 
column at the other end of the Strait ; and to restore the equi- 
librium a portion of its deeper water must be forced out as an 
under-current towards the Atlantic, thus again reducing the sur- 
face-level of the Mediterranean. Now, so long as the warm 

455 
Atlantic water which comes in to maintain that level is in its 
turn subjected to the same cooling, with consequent lowering of 
level and increase of density, so long would the vertical pressures 
of the two columns, which would be speedily restored to equi- 
librium if both basins were subjected to the same heat or the 
same cold, remain in a constant state of inequality ; and so Jong, 
therefore, must this vertical circulation continue. 
Now, the case thus put hypothetically has a real existence. 
For the Mediterranean, cooled down by the withdrawal of solar 
heat, let us substitute the Polar basin, and for the Atlantic, the 
Equatorial Ocean, The antagonistic conditions of temperature 
being constantly sustained, a constant interchange between polar 
and equatorial waters through the seas of the Temperate Zone 
must be the result. The reduction in the temperature of the 
Polar column must diminish its height whilst augmenting its den- 
sity ; and thus a flow of the upper stratum of equatorial water 
must take place towards the {poles to maintain the level thus 
lowered. But when the column has been thus restored to an 
equality of height, it will possess such an excess of weight that 
its downward pressure must force out a portion of its deeper 
water; and thus an underflow of ice-cold water will be occasioned 
from the polar towards the equatorial areas. 
The agency of polar cold will be exerted, not merely in re- 
ducing the bulk of the water exposed to it, and thereby at the 
same time lowerimg its level and increasing its density, but also 
in imparting a downward movement to each new surface-stratum 
as its temperature is reduced, whereby a continual in-draught 
will be occasioned from the warmer surface-stratum around. For 
the water thus newly brought under the same same cooling in- 
fluence will descend in its turn ; and thus, as the lowest stratum 
will be continually flowing off, a constant motion from above 
downwards will continue to take place in the entire column, so 
long as a fresh stratum is continually being exposed to the in- 
fluence of surface-cold, 
On the other hand, the agency of equatorial heat, though 
directly operating on only a thin film of surface-water, will 
gradually pump-up (so to speak) the polar water which has 
reached its area by creeping along the deepest parts of the inter- 
mediate oceanic basins. For since, as already shown, an in- 
draught of the upper stratum surrounding the polar basin must 
be continually going on, the place of the water thus removed 
must be supplied by water drawn from a still greater distance ; 
and thus the movement will be propagated backwards, until it 
affects the upper stratum of the equatorial basin itself, which will 
flow off pole-wards, bearing with ita large measure of heat. The 
cold and dense polar water, as it flows in at the bottom of the 
equatorial column, will not directly take the place of that which 
has been draughted off from the surface ; but this place will be 
filled by the rising of the whole superincumbent column, which, 
being warmer, is also lighter than the cold stratum beneath. 
Every new arrival from the poles will take its place below that 
which precedes it, since its temperature will have been less 
affected by contact with the warmer water above it. In this way 
an ascending movement will be imparted to the whole equatorial 
column, and in due course every portion of it will come under 
the influence of the surface-heat of the sun. This heat will 
of course raise the level of the equatorial column, without aug- 
menting its absolute weight ; and will thus add to the tendency 
of its surface-stratum to flow towards the lowered level of the 
polar area. But as the ser-heating extends but a short way 
down, and asthe temperature of the water beneath, down to the 
“stratum of intermixture,” is very moderate, whilst the water 
below that stratum is almost as cold as that of the polar basin, 
it is evidently in the latter that the force which maintains this 
vertical circulation chiefly originates. 
Here, then, we have a vera causa for a general oceanic cir- 
culation, which, being sustained only by the unequal distribution 
of solar heat, will be entirely independent of any peculiar dis- 
tribution of land and water, provided always that this does not 
prevent the free communication between the polar and equatorial 
oceanic areas, at their depths as well as at their surface. That 
this agency has been so little recognised by physical geographers, 
we can only attribute to the prevalence of the erroneous idea of 
the uniform deep-water temperature of 39°, of which the tem- 
perature-observations made in our expeditions of 1868 and 1869 
have shown the fallacy. Until it is clearly apprehended that 
sea-water becomes more and more dense as its temperature is 
reduced, and that it consequently continues to sink until it freezes, 
the immense motor power of polar cold cannot be apprehended, 
But when once this has been clearly recognised, it is seen that 

