174 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



ing current of air, whose area in our case is defined by a circle. Out- 

 side of this circle horizontal movements prevail exclusively ; inside of 

 it there is also the vertical movement to be considered. Therefore the 

 computations for the outer and inner regions are different. In this way 

 we obtain the following results : 



(1) The pressure increases from all sides outward from the center ; 

 the gradient increases also from the center out to the limit of the inner 

 region : thence outward it diminishes and at a great distance becomes 

 inappreciable. 



(2) The wind-paths in both regions are curved lines, logarithmic 

 spirals, which cut the isobars everywhere at the same angle or make 

 everywhere the same angle with the radial gradient. Therefore the 

 movement of the air can be considered as consisting of a current toward 

 the center and a rotation around the center, the latter in direction op- 

 posite to the hands of a watch. This departure from the gradient is of 

 different magnitudes in the outer and inner regions. For the former the 

 departure has the same value as for straight-line isobars, that is to say, 

 it depends alone upon the rotation of the earth and the friction. For 

 the inner region the departure is greater, and depends besides upon the 

 intensity of the ascending current of air. If both regions were sepa- 

 rated from each other by a geometrical cylindrical surface then the wind- 

 paths in these would not continuously merge into each other, but would 

 form an angle with each other. This of course can never occur in nature. 

 We must therefore assume a transition region in which the wind is con- 

 tinuously diverted from one into the other direction. At any rate ac- 

 curate and comparative observation of the wind direction in the inner 

 and outer region of a cyclone would be of great interest. From these 

 one could draw a conclusion as to the limitation of the ascending cur- 

 rent of air. This limit is moreover also notable because at it the winds 

 reach their greatest force. 



There are no other arrangements that have been discussed theoreti- 

 cally as yet except the straight line and the circular and nearly circular 

 forms of the isobars. 



We have as yet only spoken of the invariable systems of wind. In 

 fact however their duration is relatively short, No sooner is a de- 

 pression formed than it fills up. Furthermore the central region of 

 depression generally does not remain long in the same place but wanders 

 often with great velocity, drawing the whole system of winds with it. 

 We must look to the density of the horizontal current flowing in to- 

 wards the ascending current of air as the cause of these changes. The 

 system of winds remains unchanged only when, as has hitherto been 

 silently assumed, the temperature and density of the horizontal and 

 vertical currents are alike. If the inflowing air is warmer the depres- 

 sion increases in depth; in the opposite case it becomes shallower. 



Finally, if the inflowing air is not of the same temperature on all 

 sides, but has on the one side higher and on the other side lower 



