LOCALISED CURRENTS. 15 
conclusion that instead of there being a permanent deep layer 
ot water at 4° C., the average temperature of the deep sea in 
temperate and tropical regions is about 0° C., the freezing point 
of fresh water. 
In the atmospheric ocean, aeronauts not seldom meet with 
warm air currents flowing above others of a colder temperature ; 
while, according to a general law, the warmth of the air con- 
stantly diminishes as its elevation above the surface of the sea 
increases. 
Similar exceptions to the general rule are met with in the 
ocean. In moderate depths sometimes the whole mass of water 
from the surface to the bottom is abnormally warm, owing to 
the movement in a certain direction of a great body of warm 
water, as in the “warm area” to the north-west of the Hebrides, 
where, at a depth of 500 fathoms, the minimum temperature was 
found to be 6° C. On the other hand, the whole body of 
water is sometimes abnormally cold, as in the “cold area,” be- 
tween Scotland and Faeroe, where, at a depth of 500 fathoms, 
the bottom temperature is found to average —1° C.* The only 
feasible explanation of these enormous differences of tempera- 
ture, amounting to nearly 13° F. in two areas freely communi- 
cating with one another, and in close proximity, is that in the area 
to the north-west of the Hebrides a body of water warmed even 
above the normal temperature of the latitude flows northwards 
from some southern source, and occupies the whole depth of that 
comparatively shallow portion of the Atlantic, while an arctic 
stream of frigid water creeps from the north-eastward into the 
trough between Faeroe and the Shetland Islands, and fills its 
deeper part in consequence of its higher specific gravity. There 
ean be no doubt that similar phenomena occur in various parts 
of the ocean, and that the deep seas are frequently intersected 
by streams differing in temperature from the surrounding 
waters. 
In some places, owing to the conformation of the neighbour- 
ing land or of the sea-bottom, superficial warm and cold cur- 
rents are circumscribed and localised, thereby occasioning the 
singular phenomenon of a patch or stripe of warm and a patch 
of cold sea meeting in an invisible but well-defined line. 
* «The Depths of the Sea,” by Professor Wyville Thomson, p. 307. 
