NARRATIVE OF THE CRUISE. 
957 
the atmosphere and the sea. The sea gives up portions of its water as vapour, and the 
atmosphere in its turn gives up portions of its vapour as water ; and climates are dry or 
moist according as the balance is in favour of the one or the other side of this exchange. 
Were there no currents in the atmosphere or the ocean, there would be a constant 
distribution of moisture in the air and concentration in the sea water depending on the 
temperature, subject to diurnal and annual oscillations. This stationary state of things, 
however, is by no means what is observed : both in the ocean and the atmosphere there 
are currents of vast dimensions. The great concentrating agency is the trade winds, 
which, by virtue of their dryness, remove water from the surface of the ocean over wdiich 
they blow, while, by virtue of their momentum, they mechanically drive the concentrated 
water into the equatorial area of dilution. 
The same remarks refer in a great measure also to alterations produced by changes 
from the liquid to the solid state, and vice versa. Removal of water, whether as ice or 
vapour, causes concentration ; restoration of it causes dilution. Whether the removal is 
caused by evaporation or congelation, it is to a certain extent localised so as to produce 
areas of concentration and of dilution. 
The cruise of the Challenger lasted three years and a half, and three years of this time 
were spent between lats. 40° N. and 40° S., therefore the majority of the observations 
apply to this region. From the surface observations which were made daily when at 
sea, and from those of other observers, the accompanying coloured map has been con- 
structed. In it the density of the water at 60°F. (15 0, 56 C. is represented by various 
colours, as follows : — 
Colour. 
Density at 60° F. 
Salinity in Grammes per 
Kilogramme. 
From 
To 
From 
To 
Purple, .... 
Below 1-0250 
Below 33-713 
Light blue, 
1-0250 
255 
33-713 
34-364 
Dark blue, 
255 
260 
34-364 
35-015 
Light green, 
260 
265 
35 015 
35-665 
Dark green, 
265 
270 
35-665 
36-315 
Light red, 
270 
275 
36-315 
36-965 
Dark red, 
275 
280 
36-965 
37-615 
Yellow, .... 
Above 280 
Above 37 '6 15 
