284 Proceedings of the Royal Society 
a very important cause of the transmigration of rock, it does not 
sensibly affect the specific gravity of the sea-water, because only a 
very limited amount either of silica or of carbonate of lime can be 
in solution at any one time. The general distribution of saltness, 
as indicated by the specific gravity of the water at a common tem- 
perature of 60° F. at the surface, is shown on a chart on which 
localities of equal saltness are connected by lines, and the spaces 
between successive lines coloured distinctively. The vertical dis- 
tribution is shown in diagrams in sections along certain lines of 
route, the depths at which the same saltness is observed being also 
connected by lines. The variations of saltness at the surface on 
the lines of section are further shown by curves having for ordi- 
nates the reduced specific gravity, and for abscissae degrees of 
latitude or longitude according to circumstances. 
These diagrams show at once how great are the differences in the 
saltness of the water in different depths and at different localities 
on the surface, and it must be remembered that we are here con- 
sidering bona fide ocean water, litoral waters so much influenced by 
the drainage of the land being reserved for separate treatment. 
Looking at the chart we see that specific gravities above 1-0270 
occur in three localities only — namely, in the trade-wind regions 
of the North and South Atlantic, and of the central South Pacific. 
In the North Pacific the specific gravity was nowhere as high as 
1*0265, whereas large tracts both of the North and South Atlantic 
reached 1-0275. The greater part of the year 1873 was passed in 
the North Atlantic, and the central part was crossed on the home- 
ward voyage. On the eastern side and in the track of outward bound 
ships, the maximum of 1 -02763 was reached in latitude 24°. In the 
central part the maximum was 1-02768 in latitude 25°, and in the 
western part it was 1*02743 in 27°30'; so that between the meri- 
dians of 20° W. and 70° W. the line of maximum saltness would 
appear to run nearly due east and west, with a slight northerly in- 
clination towards the west. In the central South Atlantic the 
maximum of 1*02768 was observed in latitude 17°26', and in the 
western part 1*02783, in latitude 15°15', so that here also the line 
of maximum saltness would appear to have a slight northerly trend. 
Between these two lines the specific gravity falls off until a mini- 
mum of 1*0260 is reached at about 3° N. latitude, and following the 
