200 
THE VOYAGE OF H.M.S. CHALLENGER. 
invariable. The percentage composition of a given sample of ocean-water is, of 
course, liable to variation according to the place where and the time when it was 
collected. This holds true more especially of the volatile components, viz., for the 
dissolved nitrogen and oxygen, the merely dissolved part of the carbonic acid, and 
last, and not least, of the water which forms the bulk (some 96 per cent, or more) of 
the whole. 
Water, even at the lowest temperatures occurring on the surface of the globe, is appre- 
ciably volatile, and its volatility, as part of the sea, is not very materially diminished 
by the salts dissolved in it. Hence, from the whole of the area of the ocean, myriads 
of molecules of vapour of water are continuously being given out into the atmosphere; 
at the same time molecules previously given out are returning whence they came, the 
tendency, however, in every portion of atmosphere touching the ocean being to establish 
the maximum vapour- tension corresponding to the prevailing temperature. This vapour- 
tension is the greater the higher the temperature, and it increases more rapidly than does 
the temperature. Hence the rate at which the air takes up water from the sea is very 
jt< at in the tropics, less in our latitudes, and far less in the circumpolar regions. On 
the basis of some law of distribution of temperatures, it would be a matter of calculation 
to impure what this would lead to if the atmosphere were in a state of stagnation. 
But the atmosphere is not in such a state, and cannot be. The moist air in the equa- 
torial regions, being relatively warm and consequently light, ascends, while relatively 
• old and dry air streams into its place from the north and south : a corresponding part 
of the uppermost .stratum of the aerial ocean wells over and flows towards the poles. The 
consequence is that the greater part of the moisture taken up by the warm air of the 
tropics is not recondensed there, but is deposited as rain in the colder latitudes. Hence 
the sea must be less saline there than in the lower latitudes; but the permanence of a 
great excess of salinity anywhere is precluded by the oceanic currents. 
To map these currents accurately and determine their velocities is the most important 
problem of general oceanography, and the solution of this problem would obviously be greatly 
facilitated if we had a correct and complete representation of the contour surfaces of equal 
-.dinity. As a means towards this end, Mr. Buchanan, in the course of the Expedition, 
collected thousands of samples of ocean-water from a great variety of places and depths, 
and defined their salinity by determining their specific gravity at known temperatures. 
Hi- results are detailed and discussed by himself in his Report on the Specific Gravity 
..f Ocean-Water/ My own connection with this part of his work is but slight. All 
1 did was. firstly, to work out experimentally the mathematical relation between salinity 
and t«-ni|Mratur«- on the one hand, and specific gravity on the other, so that Mr. 
Buchanan - numbers might be reduced to a standard temperature, and be translated into 
-abilities ; and secondly, to determine the salinities of some 160 of Mr. Buchanan’s 
rhjn. Cliem. Clint I. Exp., part ii. 
