MARINE CURRENTS. 25 



consequently relatively denser than the equatorial, should in all cases plunge 

 beneath the more tepid Avaters with which they come in contact. But some, being 

 less saline, owing either to their slighter evaporation or to their mingling with 

 the fresh water of the melting icebergs, are in fact lighter than the surrounding 

 warmer masses, and consequently rise to the surface. Naturalists engaged in 

 exploring the oceanic depths endeavour to detect the course of these super- 

 imposed currents setting in opposite directions by ascertaining the temperature at 

 certain intervals aloug the line of soundings. This is one of the most delicate of 

 marine operations, the full significance of the recorded phenomena being itself at 

 times very difficult to appreciate. But by carefully comparing the results of 

 observations taken in different places they are able gradually to arrive at trust- 

 worthy conclusions. 



The normal sequence of temperatures from the surface to the bottom has already 

 been determined. The upper layer being in contact with the atmosphere, its tem- 

 perature coincides with that of the local isothermals, while the deeper waters are 

 scarcely above freezing-point, the intermediate spaces showing a regular transition 

 between the two extremes. All anomalies in this gradual transition, all abrupt 

 changes are assumed to indicate "the presence of disturbing currents. Thus in the 

 austral seas, between o^'^ and 66° south latitude, the gradation of temperature is 

 modified by the neighbourhood of floating ice. At from 500 to 1,100 feet below 

 the surface a cold layer intervenes between the upper strata heated by the summer 

 suns and the lower waters whose temperature decreases normally downwards. 

 This cold layer, which oscillates about the freezing point, is evidently due to the 

 melting of the enormous icebergs always present in these latitudes. 



Of the special cold currents either setting from the poles or rising from the 

 lower depths, the most remarkable for its influence on the climate of the coastlands 

 is the stream named from Humboldt, and known also as the Peruvian Current, 

 which skirts the western shores of South America, and which is from 20° to 

 22° F. colder than the neighbouring waters. The North American seaboard is 

 also washed by a frigid stream, which flows southward to the equatorial seas. 

 A small part of this stream may perhaps in Bering Strait intersect a branch 

 of the tepid water setting towards the Arctic Ocean ; but the great mass of the 

 cold water trending southwards comes from the Alaska seas and other inlets of the 

 North Pacific. 



The marine waters are thus being everywhere constantly displaced, and in this 

 way the southern floods with their corresponding flora and fauna are carried 

 northwards, while the regions of the torrid zone are tempered by contact with 

 the polar currents. The climates of the two different zones blend in a new 

 climate, thanks to the intermingling streams, or else flow side by side in opjDosite 

 directions, since to every displacement corresponds an opposite movement. Even 

 the great equatorial stream has its counter- stream, which answers to the atmo- 

 spheric zone of calms, and which, especially from June to October, sets in the 

 direction from west to east, that is, from New Guinea to Equador. It is precisely 

 in the axis of the equatorial stream, and especially south of the line, that this 



