DEDUCTIONS AND REMARKS. ' t39 



now exist. To both these doctrines there are, however, insuperable objections,^ 

 and a correct theory can be obtained only by combining the two, for both must 

 certainly operate, according to well-known physical laws, and unless neutralized 

 cannot fail of producing their appropriate effects. It was by thus combining them 

 that Hadley succeeded in satisfactorily accounting for the trade-winds, more than a 

 century ago, and it remains to show that the same principles may be generalized 

 so as to explain each of the three systems, which, according to our investigations, 

 exist. 



The rarefaction of the air near the equator, by heat, will cause it to rise, and give 

 place to the colder, and, therefore, heavier air of the temperate and polar regions. 

 The amount of this influence can be calculated, and it is found to be commensurate 

 with the effects observed. If no other cause then 

 existed, we should have a regular vortex, extend- n^^^ ""^""""f^ 

 ing from the equator to the pole, as represented in Ov ■^ F'^^^^^K 

 the accompanying figure, in which E represents a mp^^ \^cV^^^>v§>> 

 point on the equator; N the north pole; C the centre 

 of the earth ; B D a line drawn through the centre 

 of the vortex, and the several arrows the direction in 

 which the air moves. 



If the air were equally dense in all parts of the 

 vortex, and its velocity the same, the centre of the 

 vortex must be over that parallel of latitude which 

 bisects the northern hemisphere, viz. the parallel of 30°. South of this parallel, 

 the air must ascend, and north of it, it must descend. In point of fact, the centre 

 must be a little farther north, since the descending currents are colder and more 

 condensed than the ascending ones, and consequently must occupy less space, but 

 the difference is not material. The result would be, a constant current along the 

 surface of the earth from the poles toward the equator, while the air which ascended 

 at the equator would flow back again toward the poles. As applied to the northern 

 hemisphere, the lower current would be from north to south, and the upper from 

 south to north. 



But if we now take the rotation of the earth into account, it will modify these 

 motions. As the heated air at the equator rises, and attempts to flow northerly 

 toward the pole, it crosses successive parallels of latitude, whose easterly motion, 

 by virtue of the earth's diurnal revolution, is continually diminishing. But the 

 air, retaining the easterly motion which it had at the equator, and consequently 

 moving more rapidly in that direction than the places over vs^hich it passes, has a 

 relative motion, as from the west, which, combined with its northerly motion, carries 

 it toward the north-east, and finally toward the east. On the same principles, the 

 lower current must continually veer more and more toward the west, as it 

 approaches the equator. 



When we reflect that it is over 6,000 miles from the equator to the pole, while 

 one-half of the entire atmosphere lies within seven miles of the surface of the 



^ See Appendix. 



