no 



DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



It must be remarked that (b) defines curvature as a quantity which has a 

 definite sign. This sign depends on the direction for the positive increase of the 

 angle (sec. 155), and the positive direction along the curve 5. It is seen at once 

 that the rule of signs can be given in this form : 



An element ds of a curve has positive or negative curvature according as it determines 

 positive or negative circulation on the osculating circle (fig. 88). 



Fig. 90. Divided sheet for the determination of 

 curvature and divergence of curves. 



Z / 



-9 -3 



3 4567 



Fig. 91. Field of curvature (A) and field of divergence 

 (B) of a system of curves. 



For our charts in horizontal projection the same rule can be 'stated thus: 



Curvature will be positive or negative according as an observer who looks in the posi- 

 tive direction of the curve has the center of curvature to the left or to the right. 



