of Edinburgh, Session 1881 - 82 . 
641 
Conduction takes place in masses of water with such extreme 
slowness, that, when a large mass of water has acquired a given tem- 
perature, it may retain it for an almost indefinite period, and move 
over a vast area without losing much of its heat. 
The massive movement of the waters of the ocean is, broadly speak- 
ing, from the equator to the poles on the surface, and from poles to 
equator at the bottom — the cause of this movement being difference 
of density arising from varying temperature and saltness, and the 
direction being due to the greater specific gravity of cold than warm 
salt water.* 
The normal distribution of temperature in the great oceans, as 
found by the a Challenger” may briefly be described as follows : — 
Towards the surface the warmest water is found, the surface layers 
being greatly heated by solar radiation ; the water cools rapidly 
downwards for the first 200 fathoms, slowly down to 500 or 600 
fathoms, then very slowly to the bottom, where the minimum 
temperature is reached. This normal vertical distribution of 
temperature in the ocean is, however, by no means universal. 
It is frequently found that the minimum temperature is reached 
at several hundred or thousand fathoms above the bottom, as, for 
instance, in the eastern basin of the Atlantic extending along the 
coasts of Europe and Africa, where the temperature sinks steadily 
to 36 ’5® Fahr. at a depth of 2000 fathoms, and then this temperature 
extends to the bottom at 3150 fathoms. One or two remarkable 
instances of this abnormal distribution of temperature may be 
mentioned. 
In the Celebes Sea, which attains a depth of 2600 fathoms, the 
minimum temperature (38*6° Fahr.) is reached at 800 fathoms ; the 
Banda Sea, with a depth of 2800 fathoms, reaches its minimum tem- 
perature of 38° Fahr. at 900 fathoms ; and the Sulu Sea, which is at 
least 2550 fathoms deep, has a uniform temperature of 50*5° Fahr. 
from a depth of 400 fathoms to the bottom. World wide examples 
of the same phenomenon were found during the “ Challenger ” 
investigations. Two neighbouring stations frequently gave water 
* See papers by Prof. Tait (May 16, 1881, Proc. Roy. Soc. Edin., vol. xi. 
p. 217), and by Profs. Marshall and Smith and Mr. Omond (read before Roy. 
Soc. Edin., July 3, 1882), on the effect of pressure on the maximum density 
point of water. 
