458 Mr. W. Brown on the Itifiuence of Atmospheric Currents 



must, by the laws of hydrostatics, press upon and displace the 

 lighter ; and a current will be established from the poles to 

 the equator. 



" This difference of gravity becomes less as we ascend from 

 the surface, and at a certain point is neutralized; while, on the 

 other hand, the elasticity which is equal at the surface varies 

 with the height, and the barometer stands higher at equal 

 elevations in the equatorial than in the polar column, and, at 

 some definite height, must more than compensate the unequal 

 density of the lower strata, and occasion a counter flow from 

 the equator to the poles." 



The calculations of the opposite forces of density and of 

 elasticity, which Professor Daniell has given, it is not neces- 

 sary here to repeat; but he adds, "As we have calculated 

 that those currents, with such respective degrees of force, are 

 the consequence of the equal height of the barometer all over 

 the surface of our sphere, so we conclude that this equal height 

 is maintained by this constant and regular flow ; and any ir- 

 regularity communicated to the currents would immediately 

 be shown by a change in the mercurial column. Let us 

 imagine, for an instant, that any cause (no matter at present 

 whence originating) should retard the velocity of the polar 

 current without at first affecting the equatorial, it is obvious 

 that the barometer would fall at the equator and rise at the 

 poles, for the balance of forces would be disturbed by the want 

 of compensation for the matter removed at one extremity and 

 accumulated at the other." 



The terms polar and equatorial, when applied generally, 

 must of course be considered as merely relative, and not con- 

 fined to the two extremes of temperature ; and, as in the pre- 

 sent essay we may confine ourselves to the northern hemi- 

 sphere, they may be used indiscriminately with north and south. 



I have quoted these passages from the writings of this di- 

 stinguished philosopher, because it was necessary that these 

 principles, which are strict and evident deductions from the 

 laws of hydrostatics, should be made the foundation upon 

 which the present inquiry should be conducted, and it would 

 not have been easy to have stated them more clearly or con- 

 cisely. 



The question is now, therefore, simply resolved into this : 

 How far the regularity of these compensating currents is 

 maintained, and when, as occurs ovey the greater part of the 

 surface of the earth, the lower one is frequently interrupted, 

 what are the origin and the nature of the obstacles presented 

 to it? 



In the intertropical regions, and those a few degrees beyond 



