MR. J. PRESTWICH ON SUBMARINE TEMPERATURES. 
633 
the South Atlantic. In the Eastern division (section No. 3) the isotherms of 60° F. and 
50° F. are on a nearly uniform level from the equator to about 35° to 45° S. lat., and 
extending apparently not quite so far southward as in the Atlantic. In the Western 
division of the Pacific (section No. 4) the several isotherms seem to lie rather deeper, 
and the isotherms of 60° and 50° F. to extend some degrees further south. But we again 
have, as in the South Atlantic, the same expansion of the isotherms of 40° and 35° F. 
as they range southward, the latter having in lat. 65° S. a depth of 6000 to 7000 feet; 
from this point it rises rapidly, or is displaced by colder waters, as it approaches the 
Antarctic continent. 
Section No. 5, which crosses the Indian and Southern Oceans from 20° N. to 40° S., 
exhibits conditions analogous to those which obtain in the Pacific, though the isotherms 
of 40° and 35° appear to lie deeper, viz. at depths of about 9000 to 12,000 feet at the 
equator. They are then prolonged nearly on the same level to about 12° north, and 
thence to rise as they approach the head of the Arabian Gulf, where they are lost in 
the heated surface-waters. In the other direction the three higher isotherms on this 
line of section maintain a more nearly uniform relative depth of about 200, 500, and 
1500 feet, — that of 80° F. terminating in about lat. 20° S., that of 70° F. in lat. 30° S., 
and that of 60° F. in lat. 39° S. At this point the isotherm of 50° F. lies at a depth 
of about 1500 feet, that of 40° F. at 4000 to 5000 feet, and that of 35° F. may be at 
about 7000 to 8000 feet. In this section we have no data south of 40° S. lat. 
Section No. 6 traverses the Southern Ocean more to the eastward. We there still 
find the higher isotherms terminating in nearly the same parallels of latitude ; but we 
can follow the lines of 40° F. and 35° F. further south — the former at a depth of about 
4000 feet in lat. 53° S. and becoming lost in about lat. 65° S., and the latter rising and 
disappearing in about lat. 70° S. South of this is a zone in which the temperature 
of the sea to the depths (1800 feet) yet tried is 30° and 33° F. (corr.). 
In the preceding observations the position of the bathymetrical isotherms can only be 
taken as an approximation to the truth, though they are, there is reason to hope, rela- 
tively correct. The deeper isotherms have possibly too high a degree, and the upper 
ones, it must be borne in mind, are, in different meridians, subject to the action of 
many causes that may produce aberration, such as displacement by the action of surface- 
currents, which will vary according to their depth ; while another manifest cause, 
affecting more especially the lower isotherms, arises from the inequalities of the 
sea-bed, whereby the lower cold strata are deflected and driven nearer to the surface — 
an effect not only due to submarine banks and some islands, but caused also by conti- 
nental shores, as on parts of the southern coasts of Africa and of South America*. 
Independently, however, of these local variations, certain general conditions have 
been clearly established by the researches we have had occasion to review, — such as 
the presence of a stratum of water at and below 35° extending from the Arctic and 
* When this takes place the temperature of the sea at or near the surface will be found to become lower on 
approaching the shore, against which the colder undercurrent rises. Their existence may thus be proved. 
