i4o 



SMITHSONIAN MISCELLANEOUS COLLECTION'S 



VOL. 51 



We shall find the variation of the pressure in any other direction 

 whatever by projecting the maximum variation upon that direction, 

 which projection is geometrically represented by the chord of a 

 circle whose diameter is the maximum variation. See the fig. No. 1, 

 in which / / represents the isobar, N the maximum variation, and 

 O P the variation of the pressure in the direction O P. 



In meteorology we call the gradient the variation of the pressure 

 normal to the isobar and expressed in millimeters of mercury per 

 degree of a great circle. 



Let G be the gradient, d p infinitely slight increase of pressure, 

 d 11 an infinitely slight increase of the normal, /j. a constant, we have 



dp 



,— = /j.G 

 du 



10333 



90 



(1) 



ft 



760 



10 000 000 



= 0.00012237 



J 



The direction of the gradient is shown by that point of the com- 

 pass toward which the pressure is least. From the theory of fluids 



it is evident that the quantity - G represents the force produced by 



P 

 the variation of the pressure acting on the unit of mass. This 

 force which acts in the direction toward which the pressure dimin- 

 ishes, must be added to the exterior forces. We shall see later that 

 the gradient is a small magnitude which even in cyclones does not 

 exceed ioo mm . 



Terrestrial gravity g is equivalent at the surface of the earth to a 

 gradient of 10570 mm. 



