640 TEMPEEATUKB OF THE OCEAN. 



The variations of heat and cold, due to change of season, or to day and 

 night, which affect the surface, descend to a comparatively small depth, 

 being greatly reduced in the first 100 fathoms, and below that depth are 

 for the most part eliminated. 



The Sun's direct influence probably does not extend below 150 fathoms ; 

 below this is a band of 300 to 400 fathoms, in which the temperature falls 

 rapidly to about 38°, and thence to the bottom, which is near 32°, except 

 in inland seas and oceanic basins. Mr. Agassiz says : — "The great cold 

 M the bottom-water of the Ocean, is best brought home to those who have 

 examined the contents of a trawl under the Tropics. The bottom ooze is 

 intensely cold, and it is a strange sensation, while one's back is broiling 

 beneath a Tropical sun, to have one's hands nearly frozen while assorting 

 the contents of the trawl." 



Surface Temperature. — Speaking roughly, the Mean Annual Surface 

 Temperature of the water of the Atlantic Ocean, is about 75° F. at the 

 Equator (between 10° N. and 10° S.). Between 10° N. and 38° N. it is 70°. 

 Northward of 38° N., the Summer Isotherms (lines passing through places 

 of equal mean temperatures) of 54|°, 50°, and 45^°, turn sharply north- 

 wards to the East of the Banks of Newfoundland ; diverging one from 

 another, and from the summer Isotherm of 60°, at intervals which are 

 pretty nearly equal, almost as far to the East as the meridian of 30° W. ; 

 but then again trending strongly to the North, so that the summer Isotherm 

 of 54|° crosses the parallel of 60° N. before (by a slight trend to the South) 

 it passes through Pentland Firth. Thence, crossing the North Sea, this 

 Isotherm passes along the coast of Norway as far as Tromso (very near 

 the parallel of 70°), and then turns southwards along the land, keeping 

 within the coast-line of Eussian Lapland, and passing across the narrow 

 throat of the White Sea. 



The summer Isotherms of 50° and 45|° cross the mouth of BafiBn's Bay, 

 and then follow the curve of the coast of Greenland towards Iceland ; 

 when approaching which they turn eastwards, the line of 50° striking the 

 land on the N.W. of Iceland, while the line of 45^° passes altogether to 

 the North of the island. To the East of Iceland the Isotherms take a 

 southerly bend, apparently under the influence of a drift of ice from the 

 Polar Sea, but soon turn northwards again ; the line of 50° running nearly 

 parallel to the coast of Norway as far as the North Cape, and then turn- 

 ing southwards along the coast of Eussian Lapland, so as to cross the 

 mouth of the White Sea to the base of the Kanin Peninsula ; while the 

 line of 45^° runs parallel to this as far North as lat. 72|°, and then turns 

 southwards, still retaining the same parallelism, so as to strike the coast 

 of Eussia beyond that peninsula. Still farther North we find the summer 

 Isotherms of 41° and 36^° showing a nearly West to East direction, until 

 they have passed the meridian of 10° W., and then suddenly turning 

 northwards ; the line of 36|° passing up to the West of Spitzbergen as far 

 as 82° N., and also extending itself irregularly eastwards along the 

 parallel of 75° as far as Nova Zembla. 



The course of the Winter Isotherms of 45|^°, 41°, 36^°, and 32°, as shown 

 in Dr. Petermann's chart, is no less significant ; for they all turn sharply 

 to the North on the eastern side of the Banks of Newfoundland, cross the 



