Sentence:  If a is the <WSD>major</WSD> axis of an ellipsoid and b and c are the other two axes , the radius 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 longest axis of an ellipse or ellipsoid; passes through the two foci