CHAP. I.] INTRODUCTION. Bi) 
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 to 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° C., 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-tem- 
peratures at great depths, which were certainly trust- 
worthy within a few degrees, had indicated several 
degrees below the freezing-point of fresh water. 
The question of the distribution of heat in the sea, 
' Physical Geography ; from the “ Encyclopedia Britannica.” By 
Sir John F. W. Herschel, Bart. K.H. &. &¢., p.45. Edinburgh, 1861. 
* Atlantic Sea-bed, p. 98. 
3 Recherches sur le Maximum de Densité des Dissolutions aqueuses. 
(Annales de Chimie, tome Ixx. 1833, p. 54.) 
D 2 
