24«2 Mr. W. Brown on the Oscillations of the Barometer. 



great care : those for London and the Orkneys are taken from 

 the tables published in the Philosophical Magazine and Athe- 

 naeum, and those for Paris from the Annates de Chimie; but 

 in general I have been indebted for them to the kindness of 

 the observers themselves. I sought those from Christiania, 

 which are from the register kept under the superintendence 

 of Prof. Hansteen, in order to enlarge the field of the observa- 

 tions ; it appears however from them that their locality is too 

 far to the west to throw light on those of this country, except 

 in a few instances ; for this reason they are placed at the foot 

 of the columns, contrary to the general order of the positions 

 of the observations ; and when reference is made to the ob- 

 servations as a whole, they are never included, except when 

 specifically mentioned. 



The whole of the facts brought to light by this investiga- 

 tion may, I think, be resolved into this general principle; 

 that all winds may be ultimately referred to the action of one 

 or both of two contrary currents caused by the unequal distri- 

 bution of temperature on the surface of the earth; the one ari- 

 sing from the flow of colder and therefore denser air towards 

 warmer, in the lower regions of the atmosphere; and the other 

 from the descent to the surface of the earth of an opposite cur- 

 rent belonging to the upper regions of the atmosphere, and 

 formed by the elasticity or total weight of the atmosphere at 

 any elevation in the warmer regions, being greater than that 

 at the same elevation in colder, because of the greater height 

 of the atmospheric column in the former than in the latter; 

 the air in both being supposed of the same pressure at the 

 surface of the earth. Thus in fig. 1, Plate V., the outline 

 H b B A represents the general figure of a portion of the at- 

 mosphere, of which the temperature decreases from A to B 

 (A lying on the equatorial and B on the polar side of C, or 

 A being south and B north); the lower current of heavy air 

 will therefore set in from B towards A. But the pressure 

 being equal at the surface of the earth, and greater at any 

 equal elevation in the column H A than in b B, at some certain 

 height above A the pressure or elasticity of the air will be so 

 much greater than at the same height above B, that its force 

 will there overcome the pressure of the colder air of the 

 column Bb; and hence above this the air will flow from H A 

 towards b B. But these currents can only be maintained by 

 air descending and ascending in some part of them : let the 

 upper current descend to the earth at A, it will continue to 

 flow towards C, as represented in the figure, by the momen- 

 tum acquired in its original position. These currents are 

 the north and south winds. 



