THE VOYAGE OF H.M.S. CHALLENGER. 
38 
These, it will be seen, are all greater than the means of the live depths given above. 
The lower specific gravity of the Indian Ocean is due to the high latitudes in which 
they were observed by the Challenger. In the Atlantic, north of lat. 30° N., twenty- 
five observations give a mean of 1*0265. Hence even at the bottom of the ocean the 
North Atlantic continues to maintain a considerably higher specific gravity than any 
other ocean, and a higher temperature is carried down to the bottom. Similar results 
are shown by relatively high bottom specific gravities from Australia eastwards to long. 
150° W. between Galapagos and Panama, and to the east of Brazil, which are 
immediately connected with the higher specific gravities and accompanying higher tem¬ 
peratures at different depths of these respective regions. 
The close connection here indicated between the specific gravities and temperatures 
of the surface and the specific gravities and temperatures of the whole ocean down to the 
bottom, renders it difficult to overestimate the importance, in the further prosecution of 
this inquiry, of a correct knowledge of the mean annual specific gravity of the surface 
of the ocean, calculated from good monthly means which seldom appear in such publi¬ 
cations. It is from such means, along with the corresponding means of the annual 
temperature of the surface, that the discussion must proceed. 
