REPORT ON OCEANIC CIRCULATION. 
27 
Atlantic between latitudes 40° and 30° at depths of 1000 and 1500 fathoms are 39°‘l 
and 37°'4 ; and between the same two latitudes of the South Atlantic they are 37°'l and 
37°‘0. Hence the temperature in the North Atlantic falls 1°7, whereas in the South 
Atlantic the fall is only 0°‘l. The following gives the results for both oceans from the 
equator to lat. 40° ; — 
North 
South 
Atlantic. 
Atlantic. 
Mean Temperature at depth of 1000 fathoms, 
39°-0 
37°-8 
Mean Temperature at depth of 1500 fathoms, 
37°-3 
37°-l 
Differences, 
l°-7 
0°-7 
The relatively slow rate of decrease of temperature of the South Atlantic may be 
regarded as commencing about 800 fathoms, to be clearly marked at 1000 fathoms, from 
which depth downwards it gradually approximates through 1100, 1200, 1300, and 1400 
fathoms to what is shown at 1500 fathoms on Map 14. -The specific gravities at the 
bottom of the ocean (Map 16) afford a ready explanation of this remarkable distribution 
of temperature. Owing to the higher specific gravities of the North Atlantic, an 
extensive deep-sea current from the North to the South Atlantic, carrying a higher 
temperature with it, sets in at depths at which the influence of the surface currents is no 
longer felt, and becomes more pronounced as the depth below 1000 fathoms is increased. 
Hence the North Atlantic receives large accessions to its salinity from the South 
Atlantic through the surface currents, which the deep-sea currents again return to the 
South Atlantic. It will be seen from Map 16 that this source of movement, viz., 
differences of specific gravities, is virtually absent from the deep waters of the Pacific. 
The Gulf of Mexico. 
This sea exhibits several peculiarities as regards temperature and specific gravity 
which have important bearings on oceanic circulation. The surface temperature of the 
southern half is above 80° and the northern half under it. The specific gra^fity in the 
south-eastern portion is l‘027l, rising in November to a mean of 1’0283, falling in the 
north of the Gulf to 1'0266. The high temperature and the specific gravity of the Gulf 
of Mexico is due to the circumstance that the strons^ current enterino- it from the east 
has had, during its passage across the Atlantic, its surface long exposed to a blazing sun 
and a dry, clear atmosphere. As there is no passage to westward for the current, the 
surface waters are further delayed in the Gulf till they escape into the Atlantic, chiefly 
through the Straits of Bernini. 
