Professor Mitchell on the Trade- W'mds. 31 



time a current of cool air should arrive from the climates near 

 the poles, to replace that which has been rarefied at the equator. 

 Thus two opposite currents of air are established, one in the in- 

 ferior, the other in'the superior, part of the atmosphere. But the 

 real velocity of the air, due to the rotation of the earth, is so 

 much the less as it is nearer the pole ; it ought therefore, in ad- 

 vancing towards the equator, to turn slower than the correspond- 

 ing part of the earth, and bodies placed at the terestrial surface 

 should strike against it with the excess of their velocity, and ex- 

 perience by its reaction a resistance contrary to their motion of 

 rotation : thus, to an observer who thinks himself immovable? 

 the wind seems to blow in a direction opposite to the rotation of 

 the earth, that is, from west to east, which in fact is the direc- 

 tion of the trade- winds *."" 



As Laplace speaks doubtingly of this theory, remarking 

 merely respecting it, that it " seems to be most probable,*" we 

 may, without subjecting ourselves to the charge of overweening 

 and unreasonable presumption, proceed to discuss its claims to 

 accuracy, and state our objections to it — our objections to it as 

 a full, complete, and satisfactory theory. The cause assigned 

 by Laplace, has unquestionably a concurrent influence in the 

 production of these winds. The trade-winds are here repre- 

 sented as a secondary result of the movement of the air over- 

 hanging the higher latitudes towards the equator, that move- 

 ment being caused by the more elevated temperature of the tract 

 towards which the current is directed. We are led to inquire 

 why it is, that this current and the resulting wind are confined 

 within the limits of thirty degrees on each side of the line. Why 

 does the air not rush with as great velocity from the parallel of 

 60° towards that of 30°, as from the parallel of 30° towards the 

 equator, and produce a trade-wind within the former, as well as 

 within the latter limits, especially as both of the causes upon 

 which the trade winds are made by Hadley to depend, operate 

 with greater energy in the higher than in the lower latitudes. 



(a.) The first of these causes is the excess of the temperature 

 of the equatorial region over that of the countries lying nearer 

 to the poles — of the tract under the equator, above that under 



• Pond's Translation of the System of the World. 



