L 13 ] 



II. The Theory of the Winds. 

 By R. M. Deeley, MJnst.C.E., F.G.S* 



ALTHOUGH so far no theory which meets with general 

 acceptance has been propounded to account for the 

 movement o£ the winds, it is probable that Edmund Halleyf 

 and George Hadley J have furnished us with a basis upon 

 which such a theory may be founded. Halley attributed 

 the motion of the atmosphere to the condition of unstable 

 equilibrium brought about by temperature differences between 

 one region and another; whilst Hadley showed that, owino- 

 to the rotation of the earth on its axis, air currents directed 

 north or south suffer deflexion. Since Hadley's time it has 

 been shown that all winds are subjected to a deflectino- force 

 due to this movement. 



When Halley and Hadley propounded their views, know- 

 ledge of the actual courses of the surface winds of the 

 earth was very imperfect, and very little indeed was known 

 concerning the upper currents of the atmosphere. However, 

 Hadley was able to show that, although, as Halley had con- 

 tended, the winds move in directions which to a large extent 

 might be expected from the temperature gradients, if the 

 effects produced by the rotation of the earth be taken inta 

 consideration, then the agreement between fact and theory is 

 much more satisfactory. 



With the more detailed knowledge of the actual circulation 

 of the atmosphere which has been gained since Hadlev's 

 time, numerous difficulties have presented themselves. In 

 middle latitudes, for instance, the surface winds generally 

 blow in opposition to the surface temperature gradients. 



In 1857 James Thomson § propounded a theory which ho 

 considered brought the facts then known into agreement with 

 theory; but to do this he had to assume the existence of 

 currents in the upper atmosphere which have not been 

 found to exist. His view was that in middle latitudes there 

 were three distinct westerly currents of air : a middle 

 current from north and west, and an upper and under current 

 from south and west, the lower current being due to the 

 decrease of velocity resulting from friction with the ground 

 or water. Ferrell at a later date advocated a similar theory. 

 The investigation of the upper atmosphere by pilot-balloons 



* Communicated bv the Author. 

 t Phil. Trans, vol. xvi. No. 183, p. 153 (1686). 

 % Phil. Trans, vol. xxxix. No. 437, p. 58 (1735). 



§ Rep. Brit. Assoc. 1857, Notices and Abstracts, p. 38. Also Bakerian 

 Lecture, Phil. Trans. 1872, vol. clxxxiii. A. p. 653. 



