CIRCULATION OF THE ATMOSPHERE 439 



case air flows from the center in both directions towards the cold ends 

 of the canal in the upper levels, and from the cold ends towards the 

 middle in the lower levels. In this figure we have therefore a general 

 description of the primary motion of the air on the earth taken as a 

 whole, by which the air flows from the tropics towards the north pole 

 and the south pole of the earth, respectively, in the upper levels, and 

 from the north pole towards the tropics and from the south pole 

 towards the tropics in the lower levels. It should be remarked in 

 passing that since the assumed canal is like a rectangular square box 

 in our laboratory experiments, but as a matter of fact of a wedge shape 

 in the earth's atmosphere, the circulation is not so easy as might be at 

 first assumed. The meridians at the equator, which are one degree 

 apart, converge to a point at the poles, so that the atmosphere when 

 quiescent must be thought of as made up of a series of sectors or spher- 

 ical wedges. Now the air in running from the tropics towards the 

 poles runs from a broad end to a thin end of the wedge, and, since it 

 can not congest, a very complex circulation is set up in order to enable 

 it to escape unnatural compression. There are, however, many ex- 

 amples in the earth's atmosphere of masses of air which are arranged 

 much more nearly in the form of a rectangular box canal, as shown 

 when a long mass of cold air is pointing north and south, with a mass 

 of warm air pointing from south to north and lying east of it, while 

 another mass of cold air pointing from north to south is placed just 

 east of the mass of warm air. While these masses may not in fact be 

 very rectangular, yet we can study their action on the supposition that 

 sections through them produce figures which are practically rectangular 

 in shape. Suppose we have a warm mass lying between the cold masses, 

 then the warm mass will be higher above and also lower below than the 

 cold masses so far as their isobars are concerned. That is to say, at 

 the upper surface of the sections if you want to get a mass of air at a 

 certain density it will be necessary to go higher up in the atmosphere 

 over the warm mass than over either of the undisturbed cold masses 

 lying on the side, and furthermore, if one wants to get a mass of air 

 of the same density as that lying on the under side of the cold section, 

 it will be necessary to go down lower in the warmer mass, that is, 

 nearer the surface of the ground, in order to find it. Applying now 

 our principles of circulation, the action of gravity will tend to draw the 

 upper part of the warm mass over upon the cold masses to either side, and 

 thus tend to destroy the inequality in the elevation of the upper isobar. 

 Similarly the cold masses will tend to flow under the warm mass from 

 either side, and remove the discontinuity in the positions of the lower iso- 

 bars. Not only do these masses of warm and cold air tend to overflow and 

 underflow sidewise, but they seek to move, as it were, along the merid- 

 ians, northward and southward at the same time ; hence the long currents 



