ROTARY ACTION OF STORMS TRACY I 7 



the whirling mass itself must lose just so much velocity as the 

 fresh supply gains. By such a process the forces of the whirl- 

 wind would be rapidly exhausted, and its existence must speedily 

 cease. A stable source of momentum, adapted to originate and 

 sustain the uniform rotary movement, is still required; and it is 

 now proposed to develop such a source of momentum in the 

 forces generated by the earth's diurnal revolution. 



The velocity of the earth's surface in the daily revolution being 

 at the equator more than one thousand miles an hour, in latitude 

 6o° half as much, at the pole nothing, and varying in intermediate 

 places as their perpendicular distances from the earth's axis, and 

 the atmosphere near the ground everywhere taking in part or 

 wholly the motion of the surface it rests on, important conse- 

 quences upon aerial currents must follow. A body of air set in 

 motion from the equator northward maintains the equatorial east- 

 ward velocity, and when it passes over regions of slower rotation 

 deviates eastward from the meridian, and ultimately describes 

 over the earth's surface a curved line bearing towards the east. 

 A current of air from latitude 45 north, having a due south 

 direction, soon reaches regions moving faster to the east, falls 

 behind them and describes a curve to the west. Winds oblique 

 to the meridian are similarly affected. These familiar matters are 

 referred to here, and illustrated by fig. 1, to elucidate what follows. 



N 



W 



s 



FIG. I. 



The influence of the figure and revolution of the earth upon 

 east and west winds, must also be considered. A parallel of lati- 

 tude, being a lesser circle of the globe, and at all points equally 

 distant from the pole, necessarily describes upon the earth's surface 

 a curved line. But a direct course, due east at the commencement 

 follows a great circle and parting from the parallel reaches a lower 

 latitude. The due east course continued in a right line describes 

 a tangent to the curve of the latitude. The velocity of the earth's 

 surface at any place, by virtue of the diurnal revolution, has for 

 its direction the line of that tangent; and when the air reposing 

 over any spot is transferred to a region of diverse motion, the 



