In the _Atlantic Ocean_ the maximum surface temperature lies near the coast of South America, between Para and Cayenne, and another maximum occurs near the west coast of Africa, between Freetown and Cape Coast Castle.
The _Pacific Ocean_ shows the peculiarity that the surface temperatures on the western side are lower than those on the eastern side. Between 45° N. and 45° S. the temperature does not fall below 50°, but between those parallels and the poles it remains most always below that figure.
The warmest water is found in the _Red Sea_ where the surface temperature has been recorded as high as 90°. North of the equator the mean annual temperature is considerably above 80°, but south of it, to about the parallel of 25°, it varies from 80° to 70°.
CHEMICAL COMPOSITION, SALINITY AND DENSITY OF SEA WATER.
In this branch of inquiry great progress has been made, and sea water is now known to contain at least 32 elementary bodies. Its chief constituents are found to consist of the chlorides and sulphates of sodium, magnesium, potassium and calcium. It also contains air and carbonic acid.
The salinity and density of sea water have been investigated very thoroughly, particularly in the Atlantic. As the salinity of the sea water is an index of its density, changes in the former naturally affect the latter. The salinity has been found generally to decrease in the neighborhood of coasts, where rivers discharge their water into the sea, and it is a maximum in the trade zones, and a minimum in the equatorial rain belt. The salinity is affected by the degree of evaporation and by the frequency of rainfall, and is now recognized as an important factor in the biologic conditions of the sea.
Of the three great oceans, the Atlantic, with a salinity of 3.69 per cent., shows a slight preponderance over that of the Pacific and Indian Ocean, whose average salinity is 3.68 and 3.67, respectively.
In the trade belts the great evaporation augments the salinity, and hence, also, the density, and in the polar zones the formation of ice brings about the same result, though in a lesser degree. In the equatorial calm region the frequent rainfall diminishes salinity and density through the dilution of the salt water. Density and salinity are thus in a certain degree subject to seasonal changes.