84 THE PHYSICAL GEOGRAPHY OF THE jSEA. 



they come rolling in from sea, and, one after another, breaking 

 upon the beach. They curl over and are caught up, leaving foam 

 from their white caps behind, but nevertheless stirring up the sea 

 and mixing up its waters so as to keep them all alike. 



230. If the ordinances of nature require a constant circulation 

 The importance of and contiuual mixing up of the water in the sea, 

 lation. that it become not stagnant, and that it may be 

 kept in a wholesome state for its inhabitants, and subserve prop- 

 erly the various offices required of it in the terrestrial economy, 

 how much more imperative must they not be with the air ? It is 

 more liable to corruption than water ; stagnation is ruinous to it. 

 It is both the sewer and the laboratory for the whole animal and 

 vegetable kingdoms. Ceaseless motion has been given to it ; per- 

 petual circulation and intermingling of its ingredients are required 

 of it. Personal experience teaches us this, as is manifest in the 

 recognized necessity of ventilation in our buildings — the whole- 

 some influences of fresh air, and the noxious qualities of " an at- 

 mosphere that has no circulation." Hence, continual mixing up 

 of particles in the atmosphere being required of the winds in their 

 circuits, is it possible for the human mind to conceive of the ap- 

 pointment of "circuits" for them (§ 216) which are so admirably 

 designed and exquisitely adapted for the purpose as are those 

 which this view suggests ? 



231. As a physical necessity, the vertical circulation of the air 

 Its vertical move- sccms to be uo Icss important than its horizontal 



ments — how pro- i • i n • t /-\ i i 



duced. movements, w^hich we call wnid. One begets the 



other. The wind, when it blows across parallels of latitude — as 

 it always must, except at the equator, for it blows in arcs of great 

 circles, and not in small ones* — creates a vertical circulation 



* The tendency of all bodies, when put in motion on the surface of the earth, is, 

 whether fluid, solid, or gaseous, to go from the point of departure to the point of des- 

 tination by the shortest line possible ; and this, when the motion is horizontal, is an 

 arc of a great circle. If we imagine a partial vacuum to be formed at the north 

 pole, we can readily enough perceive that the wind for 5°, 10°, 20° of polar distance, 

 all around, would tend to rush north and strive to get there along the meridians — 

 arcs of great circles. This would be the case whether the earth be supposed to be 

 with or without diurnal rotation, or motion of any sort. Now suppose the place of 

 rarefaction to be any where away from the poles, then draw great circles to a point 

 in the middle of it, and the air all around would, in rushing into the vacuum, seek 

 to reach it by these great circles. Force may turn it aside, but such is the tend- 

 ency (§ 120). 



