862 THE DEPTHS OF THE SEA. [CHAP, VIIL. 
thermometer, and under-currents were practically 
unknown; but the limits of surface currents had 
been traced with considerable precision by observa- 
tions of the temperature of the surface of the sea, 
even when the movement was so slow as not to be 
otherwise perceptible. The amount of heat received 
directly from the sun may be taken approximately 
to depend upon latitude only, and this heat is in 
addition to the heat of the surface water derived 
from other sources, whatever these may be. Observa- 
tions of surface temperature accordingly give us the 
heat derived directly from the sun in the region, and 
the heat derived from the same source during the 
passage of the water to the region, in addition to the 
original heat of the water; if, therefore, the water of 
any region be derived from—that is to say, form part 
of—a movement of water from a polar source, and 
if the surface water of another area on the same 
parallel of latitude form part of an equatorial current, 
although in that particular latitude they receive in 
both cases the same amount of heat from the sun, 
there will be a marked difference in their tempera- 
ture. To take an extreme case; the mean tem- 
perature of the sea in the month of July off the 
Hebrides, in lat. 58° N., in the path of the Gulf- 
stream, is 13° C.; while in the same latitude off the 
coast of Labrador, in the course of the Labrador 
current, it is 4°°5 C, 
The distribution of surface temperature in the 
North Atlantic is certainly very exceptional. <A 
glance at the chart Pl. VII., representing the general 
distribution of heat for the month of July, shows 
that the isothermal lines for that month, instead of 
