462 Dr. Werner Siemens on the Conservation 



the more rapidly-moving upper current of air, but to the 

 ratio of this to the pressure corresponding to that height in 

 the curve of neutral equilibrium. We have thus a complete 

 explanation of the otherwise enigmatical magnitude of the 

 observed variations in the barometer in medium and high 

 latitudes. 



On the earth's surface the local minimum so produced will 

 draw in air from all sides, which ascends in cyclonic move- 

 ment and is finally carried away with the equatorial current. 

 It is thus here also the vis viva of the equatorial current 

 which produces and maintains the minimum, and thus also 

 puts the air into motion which streams in along the surface 

 of the ground towards the minimum ; since the maximum of 

 pressure is thus, in consequence of the geographical compres- 

 sion of the upper current, the cause of a resulting partial 

 return-current of the equatorial current, and the path which 

 this return-current describes in the upper regions gradually 

 sinking is marked by a division of low pressure on the earth's 

 surface. Maxima and minima thus stand in a causal connec- 

 tion, and therefore, as a rule, are produced simultaneously and 

 in geographical neighbourhood. 



The air-currents caused by these two in the lower strata of 

 air must therefore combine into currents which pass essentially 

 from maximum to minimum, but the direction of which is 

 altered in known manner by the earth's rotation. This system 

 of local winds must, however, finally yield to the equatorial 

 current itself when it reaches the earth's surface in its gradual 

 descent. As a rule (i. e. with insignificant blocking in the 

 upper current-bed) this will in fact not occur. The return- 

 stream which has been begun is completed by accumulation 

 upon the higher strata of the polar return-current, arid maxima 

 and minima disappear after constant current-conditions have 

 been again established in the higher strata of the atmosphere. 

 If, however, any considerable block takes place, it brings about 

 powerful maxima of pressure and a more rapid descent of the 

 equatorial return-current. Over a tract of low pressure this 

 will fall to the ground with a velocity but little diminished by 

 carrying with it relatively tranquil air, and will here produce 

 storms, which in the northern hemisphere begin as south-west, 

 in accordance with Dove's law of rotation, through west and 

 north-west, gradually becoming weaker by friction with the 

 ground and carrying on with it relatively quiescent air, finally 

 are absorbed in the prevailing return-current to the equator. 

 These storm-winds must now, by convection of neighbouring- 

 strata of air, produce cyclonic aerial movement far beyond 

 their own limits, which make it very much more difficult to 

 trace the regular course of the atmospheric disturbance. 



