400 ANNUAL OF SCIENTIFIC DISCOVERY. 



rotation about the earth's axis; and from its excess of centrifugal force, it 

 would be steadily impelled to a more southern locality. 



It is well known, that as any portion of our atmosphere is advancing to- 

 wards the equator, it must be continually deflected to the west; but it now 

 appears that this westward deflection must diminish centrifugal force, and 

 create a tendency to return to a higher latitude. From this cause atmos- 

 pheric movements in a northern or southern direction, meet with a resistance, 

 which is proportional to the square of the sine of latitude multiplied by the 

 distance to which the air has been removed from the parallel of repose, or 

 from the part of the earth's surface having its velocity. Along the parallel 

 of twenty degrees, the resistance would be only gijjjff P art of tlie force f 

 terrestrial gravity, for a change of two hundred miles in the polar distance ; 

 but it would be double that amount ten degrees further north, and be four 

 times as great in latitude 45 ; while it would be increased in an eight-fold 

 proportion in the vicinity of the poles. To remove a cubic mile of air one 

 degree from the north pole, would require about the same mechanical ex- 

 pense as the transportation of thirty-six cubic miles from the tenth to the 

 ninth degree of north latitude. 



Along the parallel of 00, the influence of the earth's rotation would be 

 sufficient to prevent an atmospheric circulation from extending beyond two 

 degrees of latitude, by the heating power of the sun, if the effects of friction 

 and other sources of irregularity were removed. A difference of two degrees 

 in the latitude of two places on the same meridian, can rarely cause a greater 

 difference than 3 F. in their mean temperature; and the air, by this change 

 of temperature, must receive an additional expansion, amounting to yJj-Q 

 part of its original volume. In a circulation confined to a range of two de- 

 grees, the air at the southern station must ascend with a force equal to y^ 

 of its own weight; but as this buoyancy must cease at an elevation of about 

 three miles, and as it is expended in moving two columns of air each one 

 hundred and thirty-eight miles in extent, the actual motive power operating 

 on the entire aerial mass, must be less than the yjyg-^ part of the force of 

 gravity. As the resistance arising from centrifugal force to such a circula- 

 tion, in latitude 60 is yy -jj-^y part of the force of gravity, it is evident that 

 the movement cannot be maintained, except in places where the presence of 

 the land gives the air considerable friction, and brings it to the velocity of 

 the latitude at which it arrives. In the vicinity of the equator the resistance 

 from the earth's rotation is very feeble, and hence the regular action of solar 

 heat, which is so inefficient in high latitudes, is capable of giving the atmos- 

 phere an uninterrupted course of two thousand miles in the torrid zone. 



If our day were double its present length, or if our atmosphere were four 

 times as extensive as it now is, regular trade winds would pursue an uninter- 

 rupted course from the greater part of the temperate zones to the equator. 

 Had the earth's axis been perpendicular to the plane of the ecliptic, our 

 atmosphere, if secured from the effects of local disturbances, would exhibit a 

 series of independent circulations, each being confined to a smaller range in 

 proportion as it was near the pole. This seems to be the case with the planet 

 Jupiter, whose belts are very wide about his equator, but become extremely 

 narrow in high latitudes. From the position of Jupiter in our system, and 

 his rapid rotation around his axis, we may reasonably infer that his atmos- 

 phere must be very extensive ; for otherwise it could not be so sensitive to 

 the motive power of solar heat. 



