302 THE DEPTHS OF THE SEA. [chap. vir. 



when close to the bottom, I was very careful in 

 examining the thermometer, but could discover no 

 other reason for it than the actual coldness of the 

 water." 



Notwithstanding these observations and, several 

 others telling in the same direction,: — such as those 

 of Lieutenant Lee of the U.S. Coast Survey, who 

 in August 1847 found a temperature of 2°-7 0. below 

 the Gulf-stream, at tlie depth of 1,000 fathoms, 

 in lat. 35° 26' N., and long. 73° 12' W. ; and of 

 Lieutenant Dayman, who found the temperature 

 at 1,000 fathoms, in lat. 61° N. and long. 40° W. 

 to be 0°-4 0., the surface temperature being 12°'5 C, 

 the impression seems to have prevailed among 

 physicists and physical geographers that salt water 

 followed the same law as fresb water, attaining its 

 greatest density at a temperature of 4° C. The 

 necessary result of this condition, were it to exist, 

 is thu.s stated by Sir John Herschel : "In very 

 deep water all over the globe a uniform temperature 

 of 39° Eahrenheit (4° C.) is found to prevail ; while 

 above the level where that temperature is first 

 reached, the ocean may be considered as divided 

 into three great regions or zones — an equatorial and 

 two polar. In the former of these warmer, and in 

 the latter colder water is found on the surface. The 

 lines of demarkation are of course the two isotherms 

 of 39° mean annual temperatui*e." Dr. Wallich 

 gives an excellent resume of this curious fallacy. 

 He says : " But whilst the temperature of the atmo- 

 sphere beyond the line of perpetual congelation goes 

 on gradually increasing, that of the water below the 

 isothermal line remains constant to the bottom. 



