VI 
THE VOYAGE OF H.M.S. CHALLENGER. 
during the past five years, and has been a peculiarly heavy piece of work. 
It involved a search for all available material, extracting the observations 
and reducing them to uniform standards, charting them thereafter on a large 
number of maps, a selection only of which are published with this Report. 
Much time was required in the preparation of the maps, Nos. I. and II., 
which exhibit the mean annual temperature and specific gravity of the 
surface of the ocean. The practice of including annual means in their 
published tables and maps of sea-temperatures and salinities has been 
hitherto seldom adopted by meteorologists, consequently it was necessary to 
calculate these means where possible, since it is from such alone that any 
satisfactory discussion of the temperature and specific gravity of the ocean 
at all depths can proceed. 
The present Report is divided into two parts, the first dealing with the 
surface of the ocean, and the second with the deeper ocean waters. The 
former gives a brief resume of those atmospheric conditions which bear on 
the question, particularly the distribution of pressure and the prevailing 
winds dependent on the pressure. Attention is pointedly drawn to the 
intertropical distribution of pressure and the area within this region where 
pressure is lowest, since the position of this area or band with reference to 
the equator appears to give the key to the solution of the problems under 
consideration, viz., whether any of the surface currents of the ocean, 
generated and maintained by the trade winds, cross the equator, bearing 
with them a high temperature and high specific gravity into another 
hemisphere than that in which they had their origin. The relation of the 
prevailing winds to the great currents of the ocean is pointed out as well as 
the relation of the rainfall to the specific gravities. The distribution of the 
temperature of the surface of the ocean and of its specific gravity is fully 
described and illustrated on the first two maps. On twenty-seven maps the 
temperatures of the sea at 10, 15, 20, 25, 30, &c., fathoms down to 200 
fathoms were entered with the view of ascertaining approximately the depth 
