440 TEE POPULAR SCIENCE MONTHLY 



of circulating air are naturally produced so that the warm and cold begin 

 to interflow among one another, as a matter of fact in very complicated 

 curves, the purpose of this being to restore the coincidence between the 

 isobars, isotherms and gravity levels, which had been disturbed primar- 

 ily by the heat of the sun falling upon the tropics of the earth. Hav- 

 ing considered thus briefly the general principles which would induce 

 circulation on a non-rotating earth, we can take up somewhat more 

 fully the effect produced upon this same circulation by the fact of the 

 earth's rotation; that is to say, we can discuss the circulation upon a 

 rotating earth. 



The Gyrations in the Atmosphere set up on a Rotating Earth 

 It will be desirable to define a few terms which occur in circulation 

 that will enable us to speak more briefly of the subject in its advanced 

 stages. Rotation will be confined to the motion of a mass of matter, as 

 the sun or the earth, which is revolving about its center. The rotation 

 of the earth takes place in 24 hours; the sun rotates on its axis in 27 

 days more or less. Revolution is the motion of a mass about a center 

 from which it is separated by a radius, as the revolution of the moon 

 about the earth, or the earth about the sun, or of an ideal particle of 

 the atmosphere revolving about a center at a variable distance. Gyra- 

 tion is a more complicated motion. It consists of the revolution of a 

 mass about its center at a given radius while the center itself is moving 

 in some direction. If the moon revolves about the earth and the earth 

 revolves about the sun, each particle of the moon will describe a series 

 of gyrations forming a looped curve which describes this motion. If a 

 particle of air in a tornado revolves about its axis while the axis is 

 moving over the surface of the earth, the particle will gyrate or form a 

 looped curve relatively to the surface of the earth. Vortex motion is 

 more complex still. A vortex may be described as consisting of a 

 series of concentric tubes. The motion of the tube is such that the 

 inner tubes revolve about the axis faster than the outer tubes. A par- 

 ticle of an inner tube has a certain velocity which is greater than the 

 velocity on an outer tube, but the velocity of the inner tube multiplied 

 by its radius is equal to the velocity of the outer tube multiplied by its 

 radius. If a particle moves from an outer tube to an inner tube in a 

 vortex it can do so only by increasing its velocity of rotation. If the 

 particle moves from the outer tube towards the inner tube and at the 

 same time ascends along the axis, the particle will move in a helix. The 

 helix may be contracting, with greater angular velocity, or expanding, 

 with less angular velocity. In the latter case the particle moves from 

 the inner tube to the outer tube of a vortex. A torque is a complicated 

 motion which applies to a mass taken as a whole. The earth is cov- 

 ered by a shell of air and its actual motion may be described as a 



