CHAPTER III. 



CURRENTS OF THE OCEAN. 



The Aqueous and the Aerial Oceans — Thermal Circulation — Vertical and Horizontal 

 Extension of the Two Terrestrial Envelopes — Parallelism between Oceanic and 

 Atmospheric Currents — Surface and Under-Currents. 



The Aqueous and the Aerial Oceans. — The aqueous 

 envelope, which, as we have seen, covers about three-fourths 

 of the surface of the solid crust of the earth to an average 

 depth of from two to three miles, is itself surrounded by and 

 everywhere in contact with another envelope termed the 

 atmosphere, which forms an " aerial ocean " covering the whole 

 surface of our planet to a depth supposed not to exceed 

 eighty miles. Whether this aerial ocean has a well-defined 

 surface like the aqueous ocean is a point which remains to be 

 settled by future research. What we know is, that the density 

 of the various strata into which it may be divided decreases so 

 rapidly that at a height or depth of 18,000 feet, or 3000 fathoms, 

 we have already left behind one-half of the mass of air of which 

 it is composed. It has also been ascertained by recent observa- 

 tions, that the proportion of aqueous vapour — upon the presence 

 of which in the air the agency of the atmosphere as a storer-up 

 of heat and moisture mainly depends — diminishes with equal 

 rapidity, and is, as far as observation goes, reduced to zero at 

 a distance of only a few miles from the earth's surface. The 

 thickness of the atmospheric layer, considered as a meteorological 

 agent, may therefore be safely reduced to five miles, or even 

 less, for the greater number of the atmospheric phenomena with 

 which we are immediately concerned take place within a distance 

 of from two to three miles from the earth's surface. 



Ever since the movements of the atmospheric air and of the 



