Extension of the Two Terrestrial Envelopes. 49 



two or three miles, is but a minute fraction of the length and 

 breadth of an oceanic basin ; so is the depth of the more active 

 stratum of the atmosphere when compared with the areas of 

 sea and land with which it is in contact. 



A due consideration of this disproportion between horizontal 

 and vertical extension leads to several conclusions of some 

 importance to the student of the phenomena of oceanic and 

 atmospheric circulation, namely : — i. That what in either system 

 of currents has been called horizontal circulation must be the 

 preponderating phenomenon, vertical circulation only occupy- 

 ing the second place. 2. That the original direction, volume, 

 velocity, temperature, and composition of a current, considered 

 as part of a system of thermal circulation, must undergo im- 

 portant modifications under the influence of the terrestrial 

 areas with which the current comes in contact — an influence 

 depending upon the distribution of land and water, the direction 

 of the mountain ranges and coast lines, the configuration of the 

 surface of the land and of the bottom of the sea, and other 

 conditions present in a given area of the earth's surface. 



3. That the currents of the ocean and of the atmosphere, while 

 obeying their original tendency as thermal currents to flow in 

 a certain direction — towards the Equator in the case of polar 

 currents, towards the Poles in the case of equatorial currents 

 — will ultimately move in the direction of least resistance. 



4. That under the influence of local conditions, the general system 

 of atmospheric or oceanic currents will resolve itself into as 

 many different systems of circulation as there are distinct areas 

 of land and water. 



Sir John Herschel says, in his Treatise on Astronomy (sec. 

 197) : "We have only to call to mind the comparative thinness 

 of the coating which the atmosphere forms around the globe, 

 and the immense mass of the latter, compared with the former 

 (which it exceeds at least 100,000,000 times), to appreciate 



