362 THE DEPTHS OF THE SEA. [chap. vhi. 



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 c- 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 



