The element EFDC is just
like the circumference of a circle which is
forming base of a cone with its vertex at N. The
force on the element EFDC by the push rays is
concentrating at N because these are traveling
towards N. Their resultant effect will be
towards centre of B, and will be
times the total strength of the rays.
Therefore, resultant force
at N by the push rays passing through the
element EFDC and striking at N: