196 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



value of the rotatory motion is positive, that is to say, directed toward 

 the east ; thence to the equator the value is negative, therefore directed 

 toward the west. 



These results can easily be combined with the conclusions of my pre- 

 vious memoir, according to which the motion of rotation can be consid- 

 ered as the sum of two terms that are of entirely different natures. Of 

 the second term it was remarked especially that the current correspond- 

 ing to it first attains sensible values at great altitudes. This therefore 

 becomes at that altitude materially larger than the above deduced av- 

 erage value. The first term gave a movement entirely confined to the 

 lower strata of the atmosphere: it is directed toward the east from the 

 pole down to 35° latitude, but directed toward the west exclusively ill 

 the equatorial zone and less in velocity than the first component move- 

 ment. The numerical computation leads to the same conclusion, since 

 X2 is small In comparison with Xi Since from 35° of latitude down to 

 the neighborhood of the equator there are two currents of opposite 

 signs flowing over each other, therefore the place where the average 

 movement of rotation is 0° will lie nearer to the equator than to 35°. 



Therefore the conclusion of W. Siemens, which gave the first stimu- 

 lus to the present investigation, has to be subjected to a modification 

 only in so far as we must consider that the westward movement of the 

 upper regions and higher latitudes has a predominance over the easterly 

 movement of the lower regions and lower latitudes, because the former 

 loses a much smaller fraction tha.n the latter of its living force in con- 

 sequence of friction. 



The vertical and meridional components Fand iV'are to be added to 

 the corresponding components that were computed in my first memoir. 

 The vertical component is positive at the equator and at the pole, it 

 therefore gives an ascending current at both places, whereas V is neg- 

 ative throughout a broad central zone. Therefore at the equator the 

 ascending current is strengthened, at the pole the descending current is 

 enfeebled. 



The meridional component N is zero at the surface of the earth at the 

 equator ; it is negative, i. <?., it is directed toward the south from thence 

 to about 24° latitude ; thence to the pole, where it is again zero, it has a 

 northerly direction. Therefore in the tropics it strengthens the equa- 

 torial current and in higher latitudes it enfeebles it. Perhaps this ex- 

 plains the occurrence of northwest winds which frequently occur in the 

 southern hemisphere between 50° and 60° south latitude. 



Finally it may be remarked that the formula above used for the dis. 

 tribution of pressure agrees still better with the observations if a third 

 term with a Gth power of cos 6 is introduced. This term would also 

 find its explanation by the analytical development, since the newly 

 found meridional current should properly be again evaluated, in order 

 to further compute the movements of rotation that are to be added 



