CHAP, i.j INTR OL UCTION. 3 5 



great difficulty in arranging an index which will 

 measure with accuracy the extremely small space into 

 which even a long column of air is compressed when 

 the pressure becomes very great. It can scarcely be 

 made available beyond 1,000 fathoms (200 atmo 

 spheres). 



We have in Sir John Herschel's ' Physical Geo 

 graphy/ 1 and in Dr. Wallich's 'Atlantic Sea-bed/ 2 

 where it is given in the fullest dstail, the doctrine of 

 the distribution of deep-sea temperature as it seems 

 to have been almost universally adopted up till the 

 time of the cruise of the ' Lightning.' It was gene 

 rally understood that while the surface temperature, 

 which depended upon direct solar radiation, the 

 direction of currents, the temperature of winds, and 

 other temporary causes, might vary to any amount ; 

 at a certain depth the temperature was permanent at 

 4C., the temperature of the greatest density of fresh 

 water. It is singular that this belief should have met 

 with so general acceptance, for so early as the year 

 1833 M. Depretz 3 determined that the temperature 

 of the maximum density of sea- water, which contracts 

 steadily till just above its freezing-point, is 3'67 C. ; 

 and even before that time observations of sea-tempe 

 ratures at great depths, which were certainly trust 

 worthy within a few degrees, had indicated severa_ 

 degrees below the freezing-point of fresh water. 



The question of the distribution of heat in the sea, 



1 Physical Geography ; from the "Encyclopaedia Britannica." By 

 Sir John F. W. Herschel, Bart. K.H. &c. &c., p. 45. Edinburgh, 18G1. 



2 Atlantic Sea-bed, p. 98. 



3 Eecherches sur le Maximum de Densitc des Dissolutions aqueuses. 

 (Annales de Chimie, tome Ixx. 1833, p. 54-.) 



