366 PERCY SLADEN TRUST EXPEDITION. 



observations the mean temperature of ■wliich is 34° "4 P., being thus 0'8 degree lower than 

 the general ocean at this depth. 



Bottom. — The bottom teniperatui-e of the ocean beyond 100 fathoms is shown on a map 

 published in 1899 *, from which it appears that at the bottom of the ocean the lines of 

 equal temperature run on the whole north and south, following the general trend of tlie 

 continents, as compared with the general east and west trend of the lines of equal 

 temperature at the surface of tlie ocean. The shallower portions of the ocean's bed 

 covered by warmer waters form narrow bands along continental shores and around 

 oceanic islands, separated from each other by wide stretches of the deeper ocean-floor 

 covered by colder waters derived from the polar and subpolar regions. The bottom 

 temperature over nearly the whole of the Indian Ocean lies between 30° and 35° F., with 

 lower readings along the border of the Antarctic continent, and higher readings along 

 the shores in tropical and subtropical regions. As instances of low temperatures 

 recorded in the Indian Ocean mention may be made of a reading of 33° '9 F. at 

 a depth of 2020 fathoms in the Bay of Bengal to the east of Ceylon, and a reading of 

 33° '5 F. at a depth of 1877 fathoms to the north-west of Madagascar. 



Salinity. 



The highest salinities of the Indian Ocean — over 36 per thousand — are found south of 

 the Equator in the anti-cyclonic region lying to the west of Australia (where the 

 descending currents of dry air induce a large evaporation), and north of the Equator in 

 the western portion of the Arabian Sea. 



The high salinity in the latter area is the result of various factors. The climate of 

 Arabia and of the adjoining regions is very arid, and the evaporation is consequently 

 high. Precipitation is slight, and there is little addition of fresh water from the land- 

 masses. Moreover, saline currents of warm water flow into the Arabian Sea from the 

 E,ed Sea and the Persian Gulf. The salinity of the Persian Gulf is high — 36'38 per 

 thousand, — and the influence of the Tigris and Euphrates water, which flows in at the 

 head of the Gulf, is limited to a small region. The salinity of the Gulf of Aden and the 

 Strait of Bab-el-Mandeb is 36 to 36-5 per thousand. In the southern portion of the Red 

 Sea it is 37 per thousand, and it quickly increases on advancing northwards, reaching 

 40 per thousand in tlie northern portion and in the Gulfs of Suez and Akaba. 

 Observations made on the salinity at various depths in the Strait of Bab-el-Mandeb and 

 in the Gulf of Aden show that there is an increase of salinity down to 200 fathoms, the 

 depth of tlie ridge separating the Red Sea from the Arabian Sea. It foUows, therefore, 

 that there is an outflowing under-current of salt water from the Red Sea to the Arabian 

 Sea. Alter passing the Strait of Bab-el-Mandeb this salter water sinks to greater depths 

 as it advances on its course, and thus imparts a higher salinity to the deeper waters of 

 the north-west of the Arabian Sea. 



In strong contrast with this region of high salinity is the area covering the eastern 

 part of the Northern Indian Ocean, in the Bay of Bengal and the Malay Archipelago, 

 where the salinity is under 34 per thousand. This low saUnity is a consequence of tlie 



* Soo Murray, " On tho Temperature of the Floor of the Ocean and of the Surface Waters of the Ocean," Geogr. 

 Journ., vol. xiv. p. 34 (1899). 



