204 Prof. J. D. Everett on the General Circulation 



the equator, while air moving from east to west, relative to the 

 earth, tends to travel towards the nearest pole, so that in both 

 these cases the tendency is to the right in the northern hemi- 

 sphere and to the left in the southern. Moreover calculation 

 shows that if the velocity of the wind be small relatively to the 

 absolute rotational velocity of the earth's surface in the neigh- 

 bourhood, the force necessary to prevent deflection has the same 

 amount in the case of motion along a parallel of latitude, as in. 

 the case of motion along a meridian. Nor is it important, in 

 this respect, to distinguish between motion along a parallel of 

 latitude and motion east or west along an arc of a great circle; 

 for the deflection due to excess or defect of centrifugal force is, 

 in ordinary cases, more than a hundred times greater than the 

 deflection from one of these paths into the other. For motion 

 along a great circle in any horizontal direction, the intensity of 

 the force necessary to prevent deflection is rigorously the same 

 for all directions, and is 



2co sin X . Vy 



co being the angular velocity of the earth's rotation, X, the lati- 

 tude, and v the velocity of the motion. 



The application of these principles to the explanation of 

 cyclones will be at once apparent to those who are familiar with 

 Taylor's explanation as adopted by Dove, Herschel, and other 

 eminent authorities, an explanation founded on the tendency of 

 north and south winds to be deflected in the northern hemi- 

 sphere to their right and in the southern to their left. The fact 

 is, that winds from all points of the compass, flowing in to a 

 centre of barometric depression, experience this tendency to de- 

 flection, and the tendency is the same for them all. Accord- 

 ingly, what actually occurs is an inflow from all sides not di- 

 rectly but spirally ; or, to put the same fact in other words, there 

 is inflow towards the centre compounded with rotation round it. 

 The rotation is produced and maintained by the pressure which 

 the inflowing components exert each to its own right ; and the 

 central depression is maintained by the pressure which the ro- 

 tating components exert to their right, that is, outward from the 

 centre ; this is for the northern hemisphere. For the southern 

 we have only to put left for right. 



The law of oblique inflow from high to low barometer is not 

 confined to storms, but characterizes the ordinary movements 

 of the air, and is manifest on the most cursory glance at the 

 charts of winds and isobaric lines contained in the quarterly 

 publications of the Meteorological Office. Indeed, judging from 

 these charts, the movement to the right is much more decided 

 than the flow down the barometric gradient. Buys Ballot's 



