Sentence:  If a is the major axis of an ellipsoid and b and c are the other two axes , the <WSD>radius</WSD> of curvature in the ab plane at the end of the axis is * * f , and the difference in pressure along the a and b axes is * * f . 
Meaning of the word marked with <WSD> tags:  the radius of the circle of curvature; the absolute value of the reciprocal of the curvature of a curve at a given point