MOVEMENTS OF ATMOSPHERE GULDBERG AND MOHN 185 



Each of these four systems of wind has its calm space at the sur- 

 face of the earth which represents the interior part where the motion 

 of the air is nearly vertical and where consequently we do not feel 

 any wind. In the upper strata we must also find calm spaces where 

 the vertical motion changes into the horizontal motion or vice versa. 



These four systems which we have called simple systems, trans- 

 port masses of air, either from the surface of the earth to the upper 

 strata, or from the upper strata to the surface of the earth. When 

 we consider the case in which two or several simple systems exist 

 simultaneously so that their motions encroach upon each other and 

 the masses of air pass from above to below and inversely, we have 

 a composite system of winds of which nature offers an infinite 

 number of examples. 



Chapter V 



INTERNAL FRICTION 



§21. Horizontal currents of air of small extent 



We shall at first consider horizontal currents so small that we can 

 neglect the effect of the rotation of the earth; we also assume the den- 

 sity to be constant. Let A B and C D (fig. 20) be two horizontal 

 planes that enclose the mass of air; assume that the plane C D is 

 fixed and that the plane A B moves with a uniform velocity V . 

 The motion of the air will therefore proceed in horizontal strata of 



' C JO 



7i 





;s»s/s/js//?/> 





si -jr JS 



FIG. 20 



different velocities; along A B the velocity of the air may be w . 

 and along C D the velocity may be zero. Admitting the hypothe- 

 sis that the internal friction or the viscosity is proportional to the 

 difference between the velocities of any two strata, we conclude 

 that the velocity decreases proportionally to the distance z from 

 the plane A B. Let h be the distance of the two planes, the increase 



of velocity per unit of length will be — and we shall find the velocity 



h 





