442 THE POPULAR SCIENCE MONTHLY 



air moves into this circle on spirals, then there will be a certain 

 amount of the air moving perpendicular to the plane of the circle. 

 This whole action of spiral movement inward and vertical motion from 

 the plane is called a curl and it depends upon vortex laws. In a tor- 

 nado or hurricane the curl is illustrated by air which ascends as a cur- 

 rent while the air is moved inward along certain spirals. It is also il- 

 lustrated in electricity and magnetism where an electric current pass- 

 ing around the helix surrounds a magnetic field perpendicular to the 

 sections of the tube along which the electric current is flowing. Elec- 

 tric currents and magnetic fields are also related to each other by the 

 law of the curl, and this evidently goes back to the idea of the helix 

 or vertical spiral. 



We may now resume our discussion of the circulation of the air on 

 the rotating earth, repeating to some extent what has been said in de- 

 fining these special terms. Take a globe and in the tropics place an 

 arrow pointing westward between the equator and the latitude of 33° 

 both north and south. To the north of 33° place an arrow pointing 

 eastward, and in the southern hemisphere to the south of 33° place an 

 arrow also pointing eastward. These represent in a general way the 

 action of the atmosphere as consisting of two great whirls in each hem- 

 isphere, thus composing a torque on a hemispherical scale. Draw a ring 

 around the earth in latitude 33°, cutting out a section of the atmos- 

 phere. If this ring moves northward it will evidently contract, and to 

 have the same angular momentum, that is, mass energy, it must rotate 

 faster about the axis as it approaches the pole. This constitutes in a 

 way an illustration of vortex action whereby a particle passes from an 

 outer to an inner tube and consequently revolves faster about the axis. 

 Take another section south of latitude 33°, cutting out a ring of at- 

 mosphere. If this ring moves southward it must rotate slower because 

 it is moving to a region at greater distance from the earth's axis if it 

 is to retain the same momentum or energy of mass in motion. The 

 importance of these great torques in the earth's atmosphere can be seen 

 from this general fact that while the weight of the earth's atmosphere 

 taken as a whole is very great, and is, generally speaking, in vigorous 

 motion, yet the currents as a whole are so interbalanced that the mass 

 energy moving eastward is exactly equal to the mass energy moving 

 westward when the whole atmosphere is summed up. This is proved 

 by the fact that the rotation of the earth on its axis does not change 

 by the smallest fraction of a second from century to century, or at least 

 astronomers have been unable to detect any change in the period of the 

 earth's rotation so long as observations have been continued. If this 

 balance of eastward and westward momentum were not perfect, it would 

 immediately be shown by a change in the period of the rotation of the 

 earth upon its axis. 



