608 Sir Napier Shaw [March 10^ 



with a counter-clockwise motion in spiral curves. On the other hand, 

 the conception of the central area of an anticyclone is of a centre of 

 centrifugal motion, a region of repulsion ; the general area of an 

 anticyclone as a region of descending cold air which moves with a 

 clockwise motion spirally outwards. The fundamental dynamical 

 idea is that of air driven, like gas along a pipe, from high pressure to 

 low pressure, retarded by the friction of the surface, and diverted 

 from its direct object by the rotation of the earth. 



For future reference let us separate the three elements of this 

 picture, and keep them distinct. 



First, the circulation — counter-clockwise in a cyclone, clockwise in 

 an anticyclone. 



Second, the convergence across the circulation from high to low. 



Third, the convection or vertical motion which appears as ascending 

 air in the cyclone, and descending air in the anticyclone. 



According to the conception which developed on the lines of 

 Galton's words, and which found ready acceptance, the circulation is 

 incidental to the convergence, the convergence is universal, the 

 convection general. 



It is another example of the facilis descensus Averni. The very 

 simple piecing together of the three parts makes it almost obvious 

 that the third element, the convection, is the effective cause of the 

 whole dynamical process ; it is natural to regard convection as the 

 ascent of warm air in a relatively cold environment causing low 

 pressure on account of the relatively high temperature of the 

 ascending air ; and high pressure as the natural corollary of cold 

 descending air. The convergence or motion across the isobars is 

 the primary result of the motion due to the distribution of pressure, 

 and the circulation is merely the deviation from the straight path 

 caused by the rotation of the earth. The theory is quite simple and 

 quite self-contained, and it has this great advantage, that the cause 

 which it assigns for the cyclone, namely, the convection of warmed 

 air, has always been regarded as the cause of winds — it has been 

 accepted as explaining the trade winds and the monsoons, and if it is 

 also accepted as explaining the cyclone and anticyclone, which are the 

 modern meteorological names for the diverse winds of the temperate 

 latitudes, we can see in the idea a beautiful unity in meteorological 

 theory. The origin of all the winds is thereby assigned directly to 

 what we know must be their ultimate cause, namely, the warming of 

 the lowest layers of the air by the warmed surface of sea or land. 

 If we doubt its efficiency in one case there seems no good reason for 

 holding to it in the others. 



It seems a pity that an illusion which apparently does such good 

 service should be shattered, but it cannot face the facts of the 

 upper air. 



You will notice that the whole matter depends upon the idea of 

 the low pressure in the warm ascending air of the cyclone as the 



