THE 
VOYAGE OF H.M.S. CHALLENGER. 
PHYSICS AND CHEMISTRY. 
REPORT on the Specific Gravity of Samples of Ocean Water, observed 
on board H.M.S. Challenger during the years 1873-76. By J. Y. 
Buchanan, Esq., M.A., F.R.S.E., Chemist and Physicist of the 
Expedition. 
The variation in the specific gravity of the water which forms the ocean is, com- 
paratively speaking, so slight, that an instrument of considerable delicacy is necessary 
for determining it. So far as I have hitherto been able to observe, it lies between the 
extremes 1 '02780 and 1 ‘02400, the specific gravity of pure water at 4° C. being taken 
as the unit ; the results, therefore, if they are to be of any value, must be correct 
to at least the fourth place of decimals. In mentioning these extremes, it must 
be observed that they refer to ocean waters, and not to the mixtures of fresh and 
salt water to be found in bays and estuaries, where waters of all degrees of saltness may 
be found, from perfectly fresh to even much salter water than is represented by the above 
superior extreme, according to the climate of the locality. In deciding on the kind of 
instrument to be used, the hydrometer was selected as the only reliable one capable of 
being used at sea. Before starting I was in some doubt about the latter property, but it 
was evident that, in any case, the water samples had only to be stored till reaching 
harbour, when their specific gravities could be taken, if necessary, on shore, where the 
trustworthiness of the results would depend solely on the care with which the instrument 
had been constructed. It was found, however, that, except in very heavy weather, the 
observations could be satisfactorily made without storage being necessary. 
The hydrometer was made of glass. Metal instruments were rejected because they 
are liable to deformation by violence, and consequently to alteration of volume. 
(PHY3. CHUM. CHALL. EXP. PART II. 1883.) f B 1 
