CHAP. 1.] INTRODUCTION. 35 



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,'^ and in Dr. Wallich's 'Atlantic Sea-bed,'^ 

 where it is given in the fullest detail, 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 

 4° 0., 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^ 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, 



^ Physical Geograpliy ; from tlie " Encyclopaedia Britannica.'' By 

 Sir John F. W. Herschel, Bart. K.H. &o. &o., p. 45. Edinburgh, 1861. 



2 Atlantic Sea-bed, p. 98. 



^ Eecherches sur le Maximum deDensite des Dissolutions aqueuses. 

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



D 2 



