30 
THE VOYAGE OF H.M.S. CHALLENGER. 
borgoii falls to 2i)°. The cleci)cst sounding made was 2005 fathoms in lat. 68° 21' N. 
and long. 2° o' W., where the temperature observed was 29°'5. There are five positions, 
at depths e.xceeding 1500 fathoms, at which Mohn observed the temperature and specific 
gravity, 'riicy range from 1547 to 1861 fathoms, with a mean of 1688 fatlioms ; their 
mean position w;i.s lat. 60° 20' N. and long. 3° 27' E., and the mean temperature was 
20' -8, and mean specific gravity T0250. This temperature found in the deepest parts 
«»f the .Arctic Ocean is only from 2° to 3° higher than the freezing-point of sea-water 
of the above specific gravity. All the five positions underlie the region of the warm 
area, and it will be .seen that the specific gravity there ob.served is closely approximate 
to that of the surface. 
'riic area enclosed l)y the surface isothermal of 30° has virtually the same tempera- 
ture down to the bottom, except that at the greater depths it falls to 29°, and the same 
area appears to have a lower specific gravity than the rest of this ocean, judging from 
the small number of observations of specific gravity taken. The important inference 
follows that the orcat volume of the warm dense current from the Atlantic does not 
O 
contribute the water forming the north-easterly current that fills the cold area, but 
that it overspreiuls the region around where it terminates and thereafter descends 
downwards to greater depths, cooling as it descends till at about 1000 fathoms it 
l)ecomes nearly a.s cold :is the rest of the Arctic Ocean. The different specific gravities 
no doubt j*lay an important part in delaying the mixture of these two great bodies of 
water jdaced side by side in the region enclosed between Norway, Iceland, Greenland, 
and Spitzbergen. The high specific gravities observed at the bottom doubtless play an 
ini{)ortant part in maintaining the fluidity of the bottom water of the Arctic Ocean. 
The Southern Ocean. 
I’nder this hciul all })ortions of the ocean to the .south of lat. 40° S. are included. 
Eveiy where in this extensive region the temperature of the surface falls below 60°. 
The lowest isothermal is 40°, which may be taken ;is the annual mean about lat. 55° S. 
all round the globe. 'Fo the .south of lat. 55° S. no observations have been taken for any 
two or four months of the year, such as January and July, or February, May, Augu.st, 
and November, kf., from which a fairly good annual mean could be calculated. It is 
prolKil)le, however, that, reasoning from the positions of the isothermals of the air and 
the temjK*ratures at 100 fathoms, an annual mean of about 30° encircles the globe, 
near lat. 63* S. The sjKicific gravity is under 1 ‘0200 all round, falling to 1'0241 
•iIm.uI hit. 65* S., this being the lowest specific gravity observed by the Challenger. 
An reganls the atmos|)heric pressure and the j)revailing winds which necessarily 
r alt from the manner of its «listributioD, this ocean dillers e.ssentially from all other 
