498 W. von Siemens on the General 



the meridional flow of air, very appropriately called " main 

 circulation" by Dr. Sprung, on which my theory of the 

 the general system of air-currents is based, depends exactly 

 on the balance between the upward acceleration of the air in 

 the equatorial regions caused by the heating of the lowest 

 strata through solar radiation, and the loss of energy which 

 the air in motion suffers during its circulation. This main 

 circulation has in the course of thousands of years brought 

 about an admixture of masses of air, which without it would 

 have rotated with the surface of the earth on which they rest. 

 I only made use of the mathematical idea of an instantaneous 

 frictionless admixture of the strata of air of all latitudes, to 

 determine in a simple manner the direction and magnitude 

 of the motion which has existed since remote antiquity. 

 Ferrel does not start, as I do, from a main circulation which 

 continually interchanges the strata of air rotating with the 

 velocity due to their latitude, and thereby gradually intermixes 

 them, but brings about this admixture through a frictionless 

 displacement in the meridional direction of the rotating rings 

 of air of the different latitudes in a manner not very clearly 

 explained. We have here essentially the same basis of calcu- 

 lation as that underlying my idea of an admixture, and 

 Ferrel's and my calculation are therefore the same in result 

 as regards the direction of the currents of air ; on the other 

 hand, we differ essentially in our statements of the relative 

 strength of the wind north and south of the 35th degree of 

 latitude. I am quite of Dr. Sprung's opinion that neither 

 of the two theories can be considered quite correct. I look 

 upon mine indeed as nothing more than a first approximation 

 to the truth. And so in my calculations I have not taken into 

 consideration such complicating influences as the diminution 

 of temperature towards the poles, and the non-coincidence in 

 direction of centrifugal force and gravity. The latter fact, 

 the effect of which is also proved by the consideration that 

 the mass of air rotating in the higher latitudes must every- 

 where have a tendency to continue moving in the greatest 

 circles, and therefore to strive to reach the equator, would 

 bring about a diminution in the pressure of the air with 

 approach to the poles, and would therefore essentially prejudice 

 the result of the calculation of the admixture, if this tendency 

 were not compensated by other forces having an opposite 

 effect. It is not, however, these but other fundamental 

 assumptions which determine a very great difference between 

 the two conceptions and lead to very different results. One 

 of them is the assumption by Ferrel that the so-called law of 

 areas in the form of the conservation of the moment of inertia 



