THE CHALLENGER 
345 
as the depth increased the wealth of animal life dimin- 
ished rapidly ; and at the time in the “ forties ” and 
“ fifties ” of last century when the records of the British 
Association bristle with Committees to promote the 
exploration of the British Seas by means of the dredge, 
that attractive expression “ the bathymetric zero of life ” 
figured not infrequently on scientific pages. There was 
some reason for it, as there is for most attractive errors. 
In the Black Sea for instance, death reigns below the 
depth of a few hundred fathoms, and the mud at the 
bottom is putrid, exhaling sulphuretted hydrogen. More- 
over it had been supposed in the beginning of the nine- 
teenth century that the temperature of the sea, diminishing 
rapidly, as it was known to do, must fall so low in great 
depths as to leave the bottom covered with eternal ice. 
Observations carried out with faulty thermometers led 
later explorers to the equally false and much less logical 
conclusion that the great mass of the ocean below the 
upper skin of warm or cold water had a uniform tempera- 
ture of 39 0 F. right down to the bottom. This delusion 
may possibly have originated in some person, whose opin- 
ion carried too much weight to be lightly questioned, for- 
getting or never learning that on being cooled down, salt 
water, unlike fresh water, does not attain its maximum 
density some degrees above the freezing point. Be that 
as it may, the construction of thermometers adequately 
protected against the enormous pressure of great depths 
set the error right soon after the discovery was made that 
long submarine cables could be used for transmitting 
telegraphic messages across the oceans. 
The contour of the bed of the North Atlantic was soon 
felt out by lines of close and accurate soundings. Cables 
that had been deposited for some years in very deep 
