318 THE DEPTHS OF THE SEA. [chap, vii 



1,000 fathoms 3= ■ 5 C. 



1,250, „ 3-3 



1,476 „ 2-7 



We have here on a large scale, as Dr. Carpenter 

 has pointed out, conditions very analogous to those 

 which exist in comparatively shallow water, and on a 

 small scale in the cold area in the Pseroe Channel. 

 There is a surface layer of ahout 50 fathoms, super- 

 heated in August by direct solar radiation, and, as we 

 see by the variations of surface isothermals, varying 

 greatly with the seasons of the year. Next, we have a 

 band extending here to a depth of nearly 800 fathoms, 

 in which the thermometer sinks slowly through a 

 range of about 5° C. Then a zone of intermixture 

 of about 200 fathoms, where the temperature falls 

 rapidly, and finally a mass of cold water from a depth 

 of 1,000 fathoms to the bottom, through which, what- 

 ever be its depth, the thermometer falls almost im- 

 perceptibly, the water never reaching the dead cold 

 of the Arctic undercurrent in the E^roe Channel, 

 and the lowest temperature being nniversally at the 

 bottom (Fig: 58). 



The area investigated during the second cruise of 

 the ' Porcupine ' at the mouth of the Bay of Biscay, 

 about a couple of hundred miles west of Ushant, 

 may be regarded as simply a continiiation southwards 

 of the tract between Scotland and Ireland and the 

 Bockall vidge. As, however, the depths were greater 

 than any attained on any former occasion — were so 

 great, indeed,, as probably to represent the average 

 depth of the great ocean basins — it may be well to 

 describe the methods of observation and the condi- 

 tions of temperature somewhat in detail. 



