176 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 51 



equilibrium find a vertical gradient whose value will exceed 

 io.ooo mm provided that we use the millimeter and the degree of 

 meridian as our units. It is evident that from this definition we do 

 not get a clear idea of the force that acts during the vertical motion 

 and which must be represented by the vertical gradient. 



FIG. 15 



777. 



800 o 



7 

 6 



SO OO 



<¥■ 



3 

 2 



/OOO 

 



777 777. 

 300 



SOO 



600 



700 

 y6o 777. 777. 



fig. 1 6a 



Let the pressure be p at the height z and q the weight of the column 

 of air below z and we get 



n = p + q. 



(1) 



If the air is in equilibrium, the values of IJ will be equal to the 

 pressure p at the surface of the earth and consequently IJ will be 

 constant and independent of the height z. If the air has a vertical 

 motion the value of IJ will be different from p and will vary with 



