Abstract: In dealing with calculation of the normal potential velocity induced by a source polygon there Has a known result derived by Hess and Smith[1] in terms of local coordinates of the polygon's corners and the point to be considered under a coordinate axis system located in the polygon. The present method in terms of their global coordinates is an alternative and extension to it. Hence, there is no need to transform coordinates of points and integrate ∫∫(1/r)dS, etc. numerically in the calculation.