958 
THE VOYAGE OE H.M.S. CHALLENGER. 
The first glance at this Chart shows the coincidence of the regions of concentration 
in the sea with those of the trade winds in the atmosphere, which in their turn coincide 
closely with the areas of maximum barometric pressure. On both sides of each of these 
regions the concentration diminishes and passes into areas of decided dilution, and also 
of diminished barometric pressure. 
As the concentration of the sea water depends on the climate to which it is exposed, 
and as that is subject to certain variations, so the areas occupied by the various colours on 
the map will be subject to oscillations ; hence properly there should be similar charts 
for every month of the year ; for this purpose, however, many more observations are 
required than are at present available. The colours on the Chart will probably not differ 
very greatly from the mean positions of the regions which they indicate. To take only 
one instance, it is quite certain that the equatorial area of dilution will have a yearly 
oscillation corresponding to that of the equatorial calms ; in the eastern seas too, where 
for one half of the year a dry trade wind is blowing, and for the other a moist monsoon, 
the state of the sea water may be expected to show great variations, and these variations 
are shown in a very marked manner in the observations made in the China Sea and 
neighbouring seas, which were traversed in one direction at the end of the southwest 
monsoon when the water was comparatively fresh, and in the other direction after the 
northeast monsoon, or true trade, had been blowing for some time. The average 
density observed in the China Sea in the beginning of November was 1 ‘02510, while 
in the month of January it was 1 ‘02534. 
Taking the surface observations, it will be found that in the. North Atlantic the specific 
gravity increases from all sides up to a maximum about lat. 22° N. and long. 40° W. 
In this they agree with those of Lenz and the German ship “ Gazelle.” It is an 
opinion, expressed by Lenz and by other travellers and navigators, that the specific 
gravity of the surface water of the North Atlantic is greater on the west and less 
on the east side ; and this opinion is derived from a consideration of the observations 
on outward-bound and homeward-bound ships. The former keep close to the eastern 
margin of the North Atlantic, whereas the latter keep well out, passing usually to 
the westward of the Azores ; and it is true that the water in the centre of the North 
Atlantic, between the parallels of 15° and 30°, is denser than on the eastern side, but 
it is also denser than on the western side. 
East. 
Middle. 
West. 
Buchanan. 
Lenz. 
Buchanan. 
Lenz. 
Buchanan. 
Maximum, . 
1-02755 
1-02720 
1-02778 
1-02776 
1-02739 
Latitude of Maximum, 
24° 
31° 
23° 
20° 
27 ° 
