250 Mr. W. Brown on the Oscillations of the Barometer. 



may cause a wind in any direction, according to their force 

 and their inclination to one another; if it be on the eastern 

 side from any point of the compass between a north-eastern 

 point and some point between south and west, it will be very 

 liable to change; for winds from these points being produced 

 by the direct collision of the descending current with that 

 from north-east, when the latter is blowing on the surface of 

 the earth from some more northern station, require the conti- 

 nuance and stability of a current whose direction, inasmuch 

 as it is from east, depends upon its actual velocity, and which, 

 in some parts of it at least, must be more or less interfered 

 with by the flowing of the resulting wind; hence the winds 

 from due south, or south of east, are the most inconstant and the 

 least frequent of all the winds. But the case is very different 

 with the winds on the contrary or western side of the compass ; 

 for as the direction of the southerly current is formed in the 

 upper regions of the atmosphere, and consequently is not in- 

 terfered with by that of the resulting wind below, and the op- 

 position of the air which ought to form the northerly current 

 being simply that of a pressure from north when not actually 

 flowing towards the south, and only in some degree affected 

 by the rotation of the earth when the wind is north-west, the 

 conditions which are necessary to produce the westerly winds 

 are much more capable of giving them stability and duration. 

 When these winds, instead of meeting with their forces di- 

 rected more or less obliquely to each other, meet in direct 

 opposition with nearly equal strength, a calm or very light 

 wind is the consequence. 



12. But a rarefaction of the air being anywhere produced, 

 the direction of the wind may be further modified by the situ- 

 ation of this rarefaction, with regard to the atmospheric co- 

 lumns adjoining it, where the rarefaction does not exist; thus 

 as storms in high latitudes move towards east (§ 15), the re- 

 turning current in some parts of the storm is deflected from 

 west. 



13. The cause of these currents being the difference of 

 temperature of adjacent portions of air, their force will depend 

 upon the amount of this difference ; hence it is much greater 

 in winter than in summer, when the great length of day in 

 high latitudes lessens very greatly this difference*; hence also 

 as the degree *to which the reduction of the atmospheric pres- 

 sure can be carried by the flowing of the upper current (§ 2) 

 depends on the force of that current, and the degree in which 

 air can be accumulated (§ 7) likewise depends on that of 



* See Phil. Mag. S. 3. vol. xx. p. 467, "On the Oscillations of the Ba- 

 rometer." 



