upon the Height of the Barometer. 459 



them, these currents flow with great constancy ; and as the 

 decrease in temperature in those latitudes, in receding from 

 the equator, is small and comparatively i-egular, they are so 

 nicely balanced that the oscillations of the barometer are 

 within very small limits. But beyond these regions the con- 

 stancy of the currents is not maintained, and the oscillations 

 of the barometer increase. We are not less sure, however, 

 that the same tendency of the air to flow from colder to warmer 

 regions, which is manifested between the tropics, must always 

 exist, and that therefore there must be some force by which the 

 atmospheric current is impelled in the contrary direction. 



The origin of this opposing force is universally acknow- 

 ledged. Sir John Herschel has shown, in his ' Treatise on 

 Astronomy*,' that it arises out of that grand system of atmo- 

 spheric currents, to which, where the constancy of their course 

 is maintained, the trade-winds belong. On that latitude where 

 these winds cease, and in all latitudes between it and the pole, 

 the air of the upper or equatorial current descends, as it must 

 do, to supply the place of that flowing to the equator in the 

 north or north-east wind. But it does not descend as a simply 

 falling body ; it has acquired, in flowing from the equator, a 

 momentum, which, together with its greater relative velocity of 

 revolution, causes it to advance along the surface of the earth 

 in a direction from south-west, thus overcoming, by its im- 

 petus, the force of gravity which would urge on the north 

 wind, as the maintenance of the equilibrium requires. 



It is not my purpose at present to enter on the consideration 

 of the various pha;nomena presented by the winds, upon which 

 much light has been thrown by Professors Dovef and Kamtz %, 

 more especially by the observations of the former. It is suf- 

 ficient that it has been shown by these philosophers, that every 

 direction of the wind (excepting, of course, that depending on 

 merely local causes), even when most inconstant, may be re- 

 ferred to the action of these two currents. In speaking therefore 

 of their effect upon the pressure of the atmosphere, Tt will suf- 

 fice to consider, that whatever deflection the wind may have 

 from east or west, as it blows from the north or south of these 

 points, the polar or equatorial current predominates, and its 

 effects are due to that current. 



Here, then, we are presented with an obstacle to the influx 

 of the air from the polar to the equatorial regions, and hence, 

 when the force of the polar current is unable to overcome it, 

 there will be an accumulation of air towards the pole, and a 



♦ (Lardner's Cyclopaedia, p. 131.) 



t Phil. Mag. 1837. % Edinb.jNew PhiloBophical Journal, vol. xxvi. 



