260 
portant a matter in connection with the distribution 
of animal life and the other conditions of the problem 
at present more especially before us, and there are still | 
so many wide differences of opinion with regard to 
it, even among competent authorities, that, at the risk 
of repeating a good deal that you know already, | will 
explain to you as simply as I can what appears to me to 
be the present state of knowledge with regard to it. ‘This 
must be considered, however, merely an outline sketch, and 
when a phenomenon is represented as a sole cause or a sole 
result, I mean simply to convey that it is a cause or result 
so paramount as to reduce all accessories to insignificance. 
The ultimate source of the Gulf Stream is undoubtedly, 
as has been specially insisted upon by Sir John Herschel, 
the equatorial current of the Atlantic, the drift of the trade 
winds. The path of that portion which trends north- 
eastwards is determined by the great initial velocity of 
the equatorial water which escapes from the Strait of 
Florija. The glory of the Gulf Stream, as it issues from 
the Strait, has been the theme of every physical geogra- 
pher; and Mr. James Croll, in a valuable paper in the 
February number of the Phzlosophical Magazine on 
Ocean Currents, has entered into a careful examination of 
the actual amount of heat conveyed by the Gulf Stream 
from the Tropics into Temperate and Arctic regions. Mr. 
Croll caleulates the Gulf Stream as equal to a stream of 
water fifty miles broad and 1,000 feet deep flowing at a 
rate of four miles an hour, consequently conveying 
5,575,080,000,000 cubic feet of water per hour, or 
13 3,816,3 20,000,000 cubic feet per day. 
This mass of water has a mean temperature of 18° C. as 
it passes out of the Gulf, and on its northern journey it 
is cooled down to 4'5°, thus losing heat to the amount of 
135° C. The total quantity of heat, therefore, transferred 
from the equatorial regions per day amounts to something 
like 154,959,300,000,000,000,000 foot-pounds. 
This is nearly equal to the whole of the heat received 
from the sun by the Arctic regions, and reduced by a half 
to avoid all possibility of exaggeration, it is still equal to 
one-fifth of the whole amount of heat received from the 
sun by the entire area of the North Atlantic. 
The basin of the North Atlantic forms a kind of cvz/- 
de-sac, and while a large portion of the Gulf Stream water, 
finding no free outlet towards the north-east, turns south- 
wards at the Azores, the remainder, instead of thinning 
off, has rather a tendency to accumulate in the northern 
portions of the trough. We accordingly find that it has 
a depth off the west coast of Ireland of at least 4,800 feet, 
with an unknown lateral extension. There are no data as 
yet to determine the rate of the branch of the Gulf Stream 
which sweeps round the coast of Western Europe and into 
the Arctic Sea, but it must be very slow, for even so far 
south as at lat. 42° N. it has lost all effect upon navigation, 
its character as a constant current being entirely masked 
on the surface by the drift of the anti-trades, which has 
nearly the same direction. 
The Gulf Stream is thus a constant “river” of hot 
water, forced into a particular direction by the rotation 
of the earth, by the constant winds, and by the configu- 
ration of the land ; and accumulated and modelled by the 
confined basin of the North Atlantic and Arctic Sea. The 
cold water which replaces it is supplied under very 
different conditions. 
Sea water increases steadily in density as the tem- 
perature falls till it reaches its freezing point, about 
3° C.; the coldest water, therefore, lies at the bottom, 
and if over any region warm water be removed by 
any cause from the surface, as for instance in the 
case of the equatorial current and the Gulf Stream, 
its place will be supplied by a general indraught beneath 
of water from the coldest and heaviest, and consequently 
usually from the deepest sources from which it can be 
brought in by gravitation. The cold water is, however, 
merely drawing in to supply a vacancy, and there is no 
NATURE 
[Fuly 28, 1870 
special reason why it should follow one ingress rather 
than another, From the low initial velocity of polar 
water it will tend to flow westwards in passing into lower 
latitudes, but that tendency will probably be entirely sub- 
ordinate to specific weight in determining the course of 
the cold influx and the distribution of layers of water of 
different temperatures. 
As cold water can gravitate into the deeper parts of the 
ocean from all directions, it is only under peculiar circum- 
stances that any movement having the character of a 
current will be induced ; these circumstances occur, how- 
ever, in the confined and contracted communication 
between the North Atlanticand the Arctic Sea. Between 
Cape Farewell and North Cape there are only two 
channels of any considerable depth, one very narrow 
along the east coast of Iceland, and the other along the 
east coast of Greenland. The shallow part of the sea is 
entirely occupied, at all events during summer, by the 
warm water of the Gulf Stream, except at one point, where 
a rapid current of cold water, very restricted and very 
shallow, sweeps round the south of Spitzbergen and then 
dips under the Gulf Stream water at the northernentrance 
of the German Ocean. 
This cold flow, at first a current, finally a mere indraught, 
affects greatly the temperature of the North Sea ; but it is 
entirely lost, for the slight current whichis again produced 
by the great contraction at the Straits of Dover has a 
summer temperature of 7°5° C. The path of this cold 
indraught from Spitzbergen may be readily traced on the 
map by the depression in the surface isothermal lines ; 
and in dredging by the abundance of gigantic amphipo- 
dous and isopodous Crustaceans and other well-known 
Arctic animal forms. The other two Arctic currents 
along the coasts of Iceland and Greenland are likewise 
very apparent, taking a slightly western direction from 
their low initial velocity. 
But while the communication between the North 
Atlantic and the Arctic Sea itself, a second ¢z/-de-sac, is 
so restricted, limiting the interchange of warm and cold 
water in the normal direction of the flow of the Gulf 
Stream, and causing the diversion of a large part of the 
stream to the southwards, the communication with the 
Antarctic basin is as open as the day,a continuous and 
wide valley of upwards of 2,000 fathoms in depth stretching 
northwards along the western coast of Africa and Europe. 
That the southern cold water wells up into this valley 
there could be little doubt from the form of the ground, 
but here again we have curious corroborative evidence on 
the map in the remarkable reversal of the curves of the 
surfaceisotherms. The temperature of the bottom water 
at 1,230 fathoms off Rockall is 3'22° C., exactly the same 
as that of water at the same depth in the serial sounding, 
lat. 47° 38’ N., long. 12° 08’ W., in the Bay of Biscay, 
which affords a strong presumption that the water in both 
cases is derived from the same source ; and the bottom 
water off Rockall is warmer than the bottom water in the 
Bay of Biscay (2° 5’ C), while a cordon of temperature 
soundings drawn from the north-west of Scotland to 
a point on the Iceland shallow, gives no tempera- 
ture lower than 6°5° C. This entirely precludes the 
idea that the low temperature of the bottom water of 
the Bay of Biscay is due to any portion of the Spitz- 
bergen current passing down the west coast of Scotland; 
and as the cold current of the east of Iceland passes 
southwards considerably farther to the westward, as indi- 
cated on the map by the successive depressions in the 
surface isotherms, the balance of probability seems to be 
in favour of the view that the conditions of temperature 
and the slow movement of this vast mass of moderately 
cold water, nearly two statute miles in depth, are to be 
referred to an Antarctic rather than to an Arctic origin. 
The water of the North Atlantic thus consists first of a 
great sheet of warm water, the general northerly reflux of 
the equatorial current, the most marked portion of it\ 
