THE GULF STREAM. 247 
and volume of the oceanic currents. As a general rule, the tem- 
perature diminishes from the surface towards the bottom, a belt 
limited in depth (about 150 fathoms) alone being subject to 
variations due to the action of the sun. Below that, the tem- 
perature generally decreases with the depth, until we reach the 
body of water of which the temperature may in general be 
said to be uniform (about 35°)." 
As explanations of the oceanic currents, we have, first, the 
gravitation theory, which looks upon the differences of tempera- 
ture and of specific gravity of the water at the equator and poles 
as the prime cause of oceanic circulation ; next, Thomson’s the- 
ory, according to which the difference in evaporation and preci- 
pitation between the northern and southern hemispheres causes 
a consequent heaping up of water in the southern hemisphere, 
which south of latitude 50° is completely covered by water ; 
thirdly, the theory which attempts to account for the circulation 
by the vis inertia of the equatorial waters; and, lastly, the the- 
ory which considers the tradewinds and other prevailing winds 
as the principal causes by which oceanic currents are produced. 
Franklin, Humboldt, Rennell, Sir John Herschel, and Croll have 
supported this view of the origin of oceanic currents. 
Of course, until the extension of the frictional effect of winds 
to great depths has actually been measured, the last theory, 
plausible as it may appear, lacks its final demonstration. It is 
by no means proved, because there is an apparent connection in 
time between the periodic variations of the currents and of the 
tradewinds, that we must seek in the latter the only cause for the 
existence of the former. The presence of the Guinea Stream, 
the position of the region of calms in the northern and southern 
hemispheres, the diminishing force of the tradewinds as we ap- 
proach the equator, the rise of the colder strata of water to shal- 
lower depths in the equatorial than in the temperate regions, are 
phenomena which the action of the tradewinds alone does not 
seem to explain. Why may not oceanic circulation, like the 
1 As currents sink as soon as their tem- well as from the equator to the pole, a 
perature falls below that of adjoining combination of these varying elements 
waters, and as thetemperature diminishes may produce a somewhat complicated 
from the surface towards the bottom, as circulation. 
