444 TEE POPULAR SCIENCE MONTHLY 



then, is strongest at the surface and decreases upwards. The eastward 

 branch of the torque is a minimum at the surface and increases up- 

 wards. We have several times referred to the latitude of 33° north and 

 south of the equator as separating the eastward branch from the west- 

 ward branch of the torque, but it has now been indicated that at about 

 10,000 meters above the tropics the westward branch changes into an 

 eastward branch of the torque. As a matter of fact the surface which 

 separates the westward branch from the eastward branch spans the 

 tropics in an arch resting on the ground at 33° of latitude and crossing 

 the equator at 10,000 or 12,000 meters above it. Beneath this arch the 

 western torque is included with its maximum motion at the bottom; 

 above this arch with a broad base in each temperate zone rises the 

 eastward torque in which the velocity increases upward and gradually 

 overspreads the tropics in the higher elevations, the northern branch 

 reaching southward, and the southern branch reaching northward in a 

 comparatively thin shell till they touch somewhere above the equator. 

 All this circulation therefore constitutes a complex vortex which can be 

 referred to distinct mathematical laws. If the atmosphere were willing 

 to circulate in this simple manner it would not be difficult to adapt our 

 mathematical analysis to it, but unfortunately, instead of moving so 

 that the branches of this torque remain intact and retain their theo- 

 retical individuality, there is a continual interchange or passage of 

 currents from one branch to the other in a rather irregular way which 

 it will be necessary more closely to examine. 



The Circulation on the Bough Kotating Earth 

 The circulation which we have been describing might possibly be 

 set up on a perfectly smooth globe having the size and shape of the 

 earth, but the presence of continents and ocean areas, the mountain 

 ranges stretching north and south on the American and east and west 

 on the Euro-Asiatic continent, facilitate the breaking of these theoretical 

 branches of the torque into great circulating masses which interplay 

 among each other. It is evident that the Eocky Mountains of North 

 America and the Cordilleras of South America tend to stop the west- 

 ward currents in the tropics and the eastward currents in the temper- 

 ate zones. On the other hand, the Himalaya range in Asia tends to 

 hold the westward current in the tropical zone and the eastward cur- 

 rent in the temperate zone. There are thus certain places, that is, cer- 

 tain longitudes, where the currents tend to curl from the tropics into 

 the temperate zones. A conspicuous instance of this occurs in the 

 United States, where there is a continual outpouring of warm air from 

 the Gulf of Mexico over the Mississippi and central valleys of the 

 United States. While the trade winds in the tropics tend to blow 

 from the northeast, it is known that immense masses of air move 



