REPOET ON OCEANIC CIRCULATION. 
o 
As regards the depths of the sea dealt with in this Report, the observations were 
entered in their places on the separate maps ; but it is right to state that at none of 
these depths, not even the 100 fathoms depth, are the observations sufficiently numerous 
to warrant the drawing of lines of equal specific gravity, except for comparatively 
restricted areas of any of the oceans. The standard temperature of 60 Fahr. adopted 
by Mr Buchanan in his Report has been employed, the standard density being that of 
distilled water at 39°‘2 (4° C.). In all cases where the specific gravities have been 
published in accordance with other standards, they have been reduced to the above 
standard. The specific gravities taken from the publications of the Meteorological 
Council are all average specific gravities, many of them being the means of numerous 
observations, and on this account they are the best data at present available. By help 
of the numerous specific gravities observed by Admiral Makaroff in his cruise, and pub¬ 
lished in extenso by him, good averages were also obtained for portions of the route 
traversed by the “ Vitiaz.” 
The influence of the protracted rainfall of the summer monsoon on the one hand, 
and virtually rainless weather tracks on the other, strongly impress their specific 
gravities over wide regions. The whole of the sea off Southern Asia, from Ceylon 
round to the Chinese Sea and northwards, during the summer months indicate specific 
gravities much lower than occur anywhere else on the globe in similar latitudes, being 
occasioned chiefly by the heavy rains of the summer monsoon on the coasts. 
On the other hand, all the data entered on the maps for depths of 100 fathoms and 
greater depths were simply single observations of specific gravity, and the same remark 
applies to the temperatures employed in constructing the maps. Specific gravities thus 
obtained, as well as single temperatures from the same depths, may be truly regarded 
as good annual means. 
There were really no correct observations of deep-sea temperatures till the Miller - 
Casella thermometer, and similar thermometers having their bulbs protected from 
outside pressure, came to be employed. For some time it was concluded that, even with 
bulbs so protected, it was necessary to reduce the observed readings, the rate proposed 
by Captain Davis being 0 o, 5 Fahr. for every mile of descent from the surface; and in 
accordance therewith the whole of the temperatures published in the seven Preliminary 
Reports of Ocean Soundings and Temperatures obtained by H.M.S. Challenger, 
1873-76, were corrected. 
After the return of the Challenger the thermometers used during the Expedition 
were handed over to Professor Tait, with a view to “ the reduction of the deep-sea 
observations which were made on board the Challenger, in so far as these are affected 
by pressure.” The investigation, with the results, is published in the Challenger Reports. 1 
1 Report on the Pressure Errors of the Challenger Thermometers, by Professor P. G. Tait (Narr. Ohall. Exp., 
vol. ii., Appendix A, 1882). 
