o 
THE VOYAGE OF H.M.S. CHALLENGER 
phenomena the most important is an approximately correct representation of the 
circulation of the atmosphere over the surface of the globe in all seasons of the year. 
The only way by which this knowledge can be obtained is by the geographical 
distribution of the mass of the earth’s atmosphere, as shown by mean isobaric lines, 
which, as is recognised, ultimately depend on the geographical distribution of the 
temperature and humidity of the air. Observation abundantly shows that it is the 
isobaric lines which absolutely rule the direction of the prevailing winds. 
A discussion, therefore, of all available information relative to the different 
atmospheric phenomena, with more special reference to the work of the Challenger, 
was undertaken in 1882, and the results were published in the beginning of 1890. 1 
That Report gives, in addition to the results of the Challenger observations, the mean 
diurnal variations of atmospheric pressure at 147 places in all parts of the globe; the 
mean monthly and annual pressure at 1366 places ; a similar table of mean temperatures 
at 1620 places ; and the mean monthly and annual direction of the wind at 746 places. 
Owing to the all-importance of the wind on oceanic circulation, other data were 
obtained and utilised in constructing the maps, which had been published by the 
marine and meteorological departments of the United States, England, France, Holland, 
Germany, Norway, Denmark, Italy, Mauritius, and India. A first attempt was also 
made to investigate and describe the meteorology of the ocean. 
Among the more important factors of ocean meteorology that bear more directly 
on this discussion are the large permanent anticyclonic areas over the oceans, out of 
which the winds blow in all directions towards and over all surrounding regions where 
atmospheric pressure is lower. These are the regions of the oceans where calms and 
light winds are of most frequent occurrence. Since this state of things necessarity 
implies vast accessions of air setting towards these anticyclonic regions as upper aerial 
currents, and thence slowly descending to the surface of the sea, it follows that in these 
regions the air is relatively very dry; and this being so, evaporation is very great, 
and the specific gravity of the surface water of the sea consequently also large in 
anticyclonic regions. 
An examination of the anticyclones and their relative prevailing winds on the isobaric 
maps accompanying the Report on Atmospheric Circulation, already cited, indicate that 
these regions may be regarded, in a very large degree, as the origins of the winds 
and surface currents on the sea. It is of the greatest importance to note, in con¬ 
nection with the specific gravity of the surface water, that the less space the winds 
have traversed the ocean from their origins in the anticyclonic regions, the less is the 
rainfall. And on the other hand, the greater the distance the winds have traversed the 
ocean, the more copious is the rainfall. To take examples : The isobaric lines and 
winds for July show that the regions of the globe where at this time of the year the 
1 Report on Atmospheric Circulation, based on the Observations made on board H.M.S. Challenger, and other 
Meteorological Observations, by Alex. Buchan, M.A., LL.D. (Phys. Chem. Chall. Exp., pt. v., 1889). 
