964 
THE VOYAGE OF H.M.S. CHALLENGER. 
long. 132° W. As in the other figure the light line represents densities at 60° F. and 
the black line densities at the temperature in situ. 
The course traced by the light line in the Atlantic shows very markedly the great con- 
centration in the trade wind regions and the corresponding dilution in equatorial regions. 
In the Pacific evidence is seen of materially different conditions. In the South Pacific 
the trade wind region is clearly marked, though the concentration produced is not nearly 
so great as in the South Atlantic. In the North Pacific the water between lats. 20° and 
30° N. is hardly more concentrated than the average of the equatorial water. Between 
lats. 7° and 9° N. two very remarkable dips will be observed in the curve. The water 
here had the abnormally low density of P02475. It was in the middle of the Counter 
Equatorial Current, which was running to the eastward at the rate of 54 miles per day. 
In lat. 3° N. the Equatorial Current was crossed running 70 miles per day to the west- 
ward, and its waters were comparatively dense. 
From these curves the following Table (VI.) has been compiled, showing the mean 
densities at 60° F. and corresponding salinities in grammes per kilogramme of the 
surface waters in the two oceans as observed along these routes, and computed for every 
5 degrees of latitude. 
When the densities are reduced to their values at the temperature of the water in situ, 
it is found that, excluding regions affected by ice, the density of the water at the surface 
decreases with the latitude, and, broadly, the lightest water (1‘022 to 1 , 023) is found at 
the Equator and the heaviest (1 '026 to 1'027) in colder temperate regions. In water 
below the surface it is found that the apparent anomaly of denser water overlying less 
dense water disappears. Where the concentrated water of the trade wind region overlies 
less salt water its higher temperature makes it in situ really less dense than the fresher 
water below. So that if a series of waters be taken at any Station from the surface to the 
bottom, their densities in situ will be found to increase with the depth. Occasional 
exceptions to this rule are found. 
The greatest changes of salinity per fathom of depth occur in tropical regions where 
vigorous concentration takes place at the surface, and in the first 200 or 300 fathoms 
from the surface. Here also the temperature falls sufficiently rapidly to counterbalance 
the effect of decreasing salinity. As the depth increases the changes in salinity become 
smaller and smaller, and the rate of decrease of temperature with increase of depth 
diminishes, and the actual density of the water in situ is affected chiefly by the pressure 
of the water above it. The composition, however, of the deeper water of all the oceans 
is so nearly identical that it may be assumed that their coefficients of compressibility are 
identical ; so that, except for the purpose of actually estimating the weight of a column 
of water, for all comparative purposes the effect of pressure may be neglected, as it affects 
equally all waters at the same depth. Experiments made with piezometers on the sound- 
ing line showed that the compressibility of water decreases slightly but sensibly with 
