228 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 51 



Let us consider the cyclone of §32, in which we have a = 48 . 

 The value of a is U r cos a, and by introducing hours and degrees of 

 the great circle we shall have 



ten aqo 60 X 60 X 9 Q _ 



a = 150. cos 48 . = 3.25 



10 6 



For the isotherm ab we have / (r) = 12° and we have calculated 

 its position at the end of 2, 4 and 6 hours (see the dotted lines in 

 % 43)- 



o 



FIG. 43 



Instead of determining the movement of the isotherms we can 

 study the variation of temperature and constant curves of equal 

 variation of r. In a short time the center of the cyclone passes 



from 0' to 0" (fig. 44) and we 

 will consider the mean posi- 

 tion 0. A particle of air de- 

 scribes the distance ab = d s 

 and we assume that it main- 

 tains its temperature con- 

 stant. Then at the point b the 

 temperature will be changed 

 and the increase of the tem- 

 perature d t will be equal to 

 the difference of the temper- 

 ature between the isotherms which pass through a and through b. 



Let us call the variation of the temperature per degree of a great 

 circle measured perpendicularly to the isotherm in the direction 

 toward which the temperature diminishes, the thermometric gradient. 



