302 THE DEPTHS OF THE SEA. (CHAP. VII. 
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 C. below 
the Gulf-stream, at the depth of 1,000 fathoms, 
in. lat. 35° 26 N., and long. 73° 12’ W.; andmon 
Lieutenant Dayman, who found the temperature 
at 1,000 fathoms, in lat. 51° N. and long. 30° W., 
to be 0°4 C., 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 fresh water, attaining its 
greatest density at a temperature of 4° C. The 
necessary result of this condition, were it to exist, 
is thus stated by Sir John Herschel: ‘In very 
deep water all over the globe a uniform temperature 
of 39° Fahrenheit (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 temperature.” Dr. Wallich 
gives an excellent réswmé 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. 
