90 THE MECHANICS OP THE EARTH'S ATMOSPHERE. 



For the east wind matters are reversed. Its moment of rotation is 

 increased by the friction on the earth's surface. The accelerated mass 

 of air [the ground layer] already finds itself in that position of equi- 

 librium which it has to occupy within its stratum, and can only press 

 forward equatorially along the earth's surface into the stratum lying 

 in front of it. If it is also simultaneously heated then the resulting 

 ascent takes place more slowly than would occur in a stratum of air 

 that is at rest at the bottom. 



Hence it is to be concluded that in the east wind, the change due to 

 friction is confined to the lower layer of air, and furthermore that it is 

 relatively more effective here than in the case of a west wind of equal 

 velocity. In general, the retarded layer of air will press forward to- 

 ward the equator, in the Northern Hemisphere as northeast wind. In 

 this motion it will continue to appear as an easterly wind since it is con- 

 tinually arriving at more rapidly rotating zones on the earth. The air 

 of the stratum lying above the retarded layer will, where the region is 

 free from obstruction, as at the outer border of the trade wind zone, fall 

 behind and will appear as an east wind, retaining its moment of rotation 

 unchanged and gradually pushing toward the equator will itself in its 

 turn experience the above described influence of friction. I would here 

 further remark that the water so abundantly evaporated in the tropical 

 zone also enters into the trade wind, but with the greater velocity of 

 rotation of the revolving earth and must diminish the retardation of the 

 latter with respect to the earth. 



The lower layers of the trade wind can press in under the equa- 

 torial calm zone itself only when any difference between their velocity 

 of rotation and that of the earth's surface is entirely destroyed. They 

 then blend with the zone of calms and increase its mass so that the lat- 

 ter broadens with its inclined boundary surface always higher above 

 the layer of diminishing east wind beneath it. 



Thus it is brought about that whereas below [nearer the earth's sur- 

 face] mostly continuous changes are taking place in the temperature 

 and the moment of rotation of the strata, on the other hand above, the 

 boundaries of the broadening zones of calms (that have the great mo- 

 ment of rotation that pertains to the equatorial air and which at 10° 

 latitude must appear as a strong west wind, and at 20° latitude as a 

 westerly storm), occur in direct contact with the underlying stratum 

 that has less velocity of rotation and lower temperature. Evidently 

 the upper side of this latter [lower] stratum can scarcely be changed as 

 to the quantity of its contained heat and of its moment of rotation, 

 while after the loss of its lower layer it is being pushed sidewise and 

 towards the equator. 



As I have already shown in my communication to this Academy, 

 April 23, 1868, on "Discontinuous Fluid Motions,"* such discontinuous 

 motions can continue for a while, but the equilibrium at their boundary 



* [See No. Ill of this collection of Translations.] 



