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THE GULF STREAM. 195 
ern 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 theory which 
considers the trade-winds 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, be- 
cause there is an apparent connection intime between the periodic va- 
riations of the currents and of the trade-winds, 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 regions of calms in the north- 
ern and southern hemispheres, the diminishing force of the trade-winds 
as we approach the equator, the rise of the colder strata of water to 
Shallower depths in the equatorial than in the temperate regions, are 
phenomena which the action of the trade-winds alone does not seem to 
explain. Why may not oceanic circulation, like the movements of our 
atmosphere, be dependent upon cosmic phenomena, practically inde- 
pendent of any secondary causes, and modified by them within very 
narrow limits? 
The difference in salinity of certain oceanic districts is in itself insuffi- 
cient to explain oceanic virculation; so that while the secondary causes 
referred to above are undoubtedly active as producing more or less ex- 
tensive local circulation, we seem justified in looking upon the differ- 
ences of temperature of the zones of the ocean as the principal cause of 
the general oceanic circulation. We may state, in the main, that the 
density of the ocean water is least at the equator, gradually rises 
toward the poles, and attains its maximum at 60° of latitude. For 
the sake of convenience we may call the density of the ocean as one at 
a depth of 500 fathoms, and consider the strata of water above and be- 
low as having a less and a greater density,* within very narrow limits; 
thus the watery envelope is not in a state of equilibrium. 
The most important disturbing factors of a uniform distribution of 
oceanic temperature are the continental masses which lie in the path 
of the equatorial currents. A comparison of the position of the 
oceanic isotherms of the North and South Atlantic shows a striking 
contrast in their course north and south of the equator. A. similar 
comparison between the Atlantic and Pacifie brings out plainly the 
contrast in the course of the isotherms of two oceans, fn which the 
disturbing effect is due in the one to continental masses and in the 
other to large groups of oceanic islands. i 
*Ocean water, at depths exceeding 1,000 fathoms, has a temperature of nearly 35° 
F., the temperature of greatest density. Should the water become either colder or 
warmer, it must expand; this it can not do, on account of the pressure. 
