32 
THE VOYAGE OF H.M.S. CHALLENGER. 
except that it is rather lower over the eastern parts off the coast of Australia than 
over the western parts off the coast of South Africa. 
The specific gravities of the Arabian Sea and of the Bay of Bengal are strikingly 
different. Owing to the very salt currents from the Red Sea and Persian Gulf, and 
to the extreme dryness of the climate, the specific gravity of the Arabian Sea is large. 
On the other hand, owing to the heavy rainfall during the summer monsoon, and to 
the northerly winds during the winter months inducing up welling, the specific gravity of 
the Bay of Bengal is low. For the same reasons the east coasts of Asia and the 
Eastern Archipelago are marked by a specific gravity much lower than anywhere else 
over the ocean in the same latitudes. The track of least specific gravity in the central 
part of this ocean is a little south of the equator from east to west. 
At 100 fathoms a large extent of this ocean has a temperature above the mean 
(Map 3), this region being virtually defined by the isothermal of 60°. The highest 
temperature, 70°, is near the centre of the ocean, about long. 80° E. The same high 
temperature is found a little to the east of the Strait of Bab-el-Mandeb, from which 
point eastwards it gradually falls to 60°. The lowest temperature, 56°, overspreads a 
considerable area lying east and west to the south of Ceylon. This area lies in the 
region of the meeting of the N.E. and S.E. trades, and its position is analogous to the 
low-temperature regions of the Atlantic and Pacific at this depth with respect to the 
N.E. and S.E. trades of those oceans. 
Unfortunately, at no depths north of lat. 40° S. are there any specific gravities 
available, except in the Arabian Sea and Bay of Bengal, and there only down 
to 400 fathoms. Outside the Strait of Bab-el-Mandeb the specific gravity is about 
U0280, but in the east of the Bay of Bengal it is P0259, or even lower, the tempera¬ 
tures being respectively 70° and 56°. The low specific gravities in the Bay of Bengal 
seriously prevent the diffusion downwards of the higher temperatures of the surface. 
In this region there is no doubt that upwelling takes place during the winter months 
when winds are northerly. 
The distribution of temperature is totally changed at 200 fathoms, nearly the whole 
of the ocean north of lat. 40° S., being above 50 o- l, the mean temperature of the 
oceans. The region of highest temperature, 61°, remains at the same place as at 100 
fathoms, and the lowest temperature,- 50°, is situated to the north-west of Australia, 
where also occurs at 100 fathoms a secondary minimum. The temperature is also 
high in the west of the Arabian Sea, and there the specific gravity is U0267 off Aden 
to the east of the Strait of Bab-el-Mandeb, gradually falling to U0261 near Ceylon. 
At 300 fathoms the whole ocean is above the average temperature to the north of 
lat. 40° S., the absolute highest, 55°, being near Cape Cuardafui. But the highest 
temperature in the ocean proper is 53°, the position of which is now fully ten 
degrees of longitude to the westwards of its position at 100 and 200 fathoms. This 
