CIRCULATION OF THE ATMOSPHERE 441 



torque. Take a bundle of paper rolled up about a center line and grasp 

 it in the two ends. Now twist the roll so that the ends move in oppo- 

 site directions. At some point in the middle there will be no motion 

 of the particles, while the upper particles of the paper move in one 

 direction and the lower particles move in the opposite direction. In the 

 case of the earth's atmosphere, in each hemisphere, there are two great 

 currents each constituting a torque. In the northern hemisphere the 

 northern current moves eastward and is called the eastward drift. In 

 the northern tropic the great current moves westward and is called the 

 westward drift. There are really two torques in the earth's atmosphere, 

 one belonging to each hemisphere, so that in the tropics, as a whole, 

 the movement is westward, while north of the Tropic of Cancer it is 

 eastward, and south of the Tropic of Capricorn it is also eastward. 

 Instead of the atmosphere running from the tropics towards the poles 

 in the meridional wedges, as a matter of fact it circulates in these 

 great torques. In the northern hemisphere north of the latitude of 

 33° there is a great eastward drift and between the latitudes of 33° 

 there is a great westward drift. Along the latitude of 33° approxi- 

 mately there is no general motion either east or west, and this corre- 

 sponds with the part of the paper bundle which does not get twisted 

 when the upper and lower ends are moved in opposite directions. 



When a current of air moves in any direction it tends to break up 

 into two volutes or spiral branches. If one takes a dandelion stem and 

 splits it along the center, the two pieces will curl over in opposite 

 directions and form beautiful right and left handed spirals. If a 

 warm current of air ascends from the ground and forms a cloud, it can 

 be seen that the middle of the cloud is ascending while the edges are 

 descending in a gentle spiral of an umbelliform shape. If a southerly 

 current of air moves northward it will tend to open up in two branches 

 to the right and left. If a northerly current moves southward it will 

 tend to open up in two branches to the right and left. The left-hand 

 branch of the southerly current and the right-hand branch of the 

 northerly current will tend to interlock or intercurl. The great cur- 

 rent in the tropics which moves westward, instead of proceeding due 

 west around the earth, tends to break up into two great volutes, one 

 curling into the northern hemisphere, and one curling into the southern 

 hemisphere. The word curl has several meanings, the first is that in 

 which it has already been used ; namely, a spiral rolling about a center. 

 The second is connected with vortex motion and is really a name for a 

 part of the helix action. If a mass of air moves in a spiral towards a 

 center, it is evident that it can not proceed long in this way without 

 some provision for its escape. If it moves in a spiral on a given plane 

 it must begin to escape along a line perpendicular to that plane. If a 

 circle is taken as a boundary in a given plane, and a certain mass of 



