PCOS £7+C 



184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



the two places. Obviously there tends to be, and, initially, actually 

 is, a horizontal flow of the air (that is, a wind) at each level, in 

 the direction of the most rapid horizontal decrease of pressure at 

 that level. Such winds, however, frequently last so long (hours at 

 least) that their directions are profoundly altered by a certain ob- 

 scure factor, namely, the rotation of the earth — the secret of the 

 above paradox — which is overlooked by almost everyone, and over- 

 looked simply because its effect on the shooting of a marble, the 

 pitching of a ball, and all the thousand other similar phenomena 

 with which we are intimatety familiar, is always negligible. 



It is easy to demonstrate, as may be found in many books and 

 articles, that an object moving in any horizontal direction tends so 

 strongly to turn to the right north of the Equator, and to the left 



south of it, as to exert a force, 

 against a restraint preventing 

 such deflection, given by the 

 equation, 



f = 2 m« sin<p, 

 in which m is the mass of the 

 object, v its speed, <p its latitude, 

 and to the angular velocity of 

 the earth's rotation. 



Consider, then, the effect of 



Fig. 1.— Diagram showing deflection of particle applying a horizontal push of 



of air toward the east. f * . n 



constant magnitude and con- 

 stant geographic direction to a mass of air, w, and assume this air 

 to be free from friction, as it very nearly is when appreciably above 

 the surface. Let m, figure 1, be this mass of air, initially at rest 

 with reference to the surface of the earth; let it be in the North- 

 ern Hemisphere, and let p be the push of constant magnitude and 

 constant direction, north. Immediately the mass moves it begins 

 to deflect from the north toward the east, and, owing to the curva- 

 ture of its path, introduces a small centrifugal force. A little later 

 p may be resolved, as shown, into two components, one normal and 

 the other tangential to the path of travel. The first, like the deflective 

 force and the centrifugal force, has no effect on the speed, being at 

 right angles to the direction of motion, while the second steadily 

 increases the speed, which, in turn, increases the deflective force and 

 the deviation toward the east. In the end, therefore, the component 

 of p, along the path, reduces to zero, and the direction of travel be- 

 comes exactly east. Hence winds that are continuous for even a few 

 hours always blow more or less closely along isobars; that is, at 

 right angles to, and net in the direction of the sustaining force — 

 around centers of pressure minima and maxima and not directly 

 toward or from them. 



