A paper fastener has a base piece with a pair of prongs which may be passed through a pair of keyed slots in a top piece and latched to the top piece to hold stacked papers together. Each of the data points in the database form the vertices of the Delaunay triangles and by interpolating the temporal averaged refractivity of the vertices of the triangle enclosing the geographical position to the given date and then interpolating the refractivity of the vertices to the geographical position within the triangle, the local ES refractivity of the geographical position is estimated. A predetermined number of return paths are identified for the selected signal-wire segment. The fine-division potential detecting step is repeated until there is obtained potential equal to or substantially equal to the desired preset potential, and there is set, as the maximum intensity, the laser intensity corresponding to the potential thus obtained.