REPORT FOR 1897—PART II. APPENDIX NO. 10.

651

After platting the pieture trace 000o' (of the perspective MW, fig. 24) in the ground plane by

means of the radiais 80a0, 80b0..........., the distances 80ôc, 80tf0............(fig. 25) laid

off npon S0P0, fig. 26, will locate those net lines (parallel with O000') in the ground plane which correspond to the lines dô, bfi . . . . shown in the perspective MW, fig. 24.

If we now transfer the points aQ', PQ, b0', d0', and cQ' from fig. 24 to a strip of paper, and place this upon the pieture trace 00(V, fig. 26, that the points P0 will coincide, the lines a0'A, bJB, . . . drawn parallel with 8QP0 will represent the net lines which are perpendicular to the ground line 0o0'o.

Thus the platted positions of the points A, B, C, and B are located on the ground plane by the intersections of the corresponding net lines of both Systems, as shown in fig. 26.

The points A, B, C, and B will, of course, also be bisected by the radiais 80a0, 80b0 . ,

which fact may make it more advantageous to select some other disposition of the net lines for a figure of a different shape.

When the figure has a sinuous perimeter the squares of the network should be selected of a size sufficiently small to enable the draughtsman to draw the perimeter sections falling within the squares sufficiently accurate to obtain a correct représentation of the general outline.

X. THE VANISHING- SCALE.

We had seen, fig. 26, that the radiais drawn from the so-called u foot of the station” (80) represent the directions to the points A, B, C . . . . in the ground plane, and if we eould détermine the distances 8aA, 80B .... (from the foot of the station 80 to the points to be

platted A, B,.............) from the perspective in some manner the location of the platted

positions in the ground plane would become an easy matter.

The distances 80A, 80B . ...... fig. 26, may be determined from the perspective by

means of the so-called vanishing scale, which may be constructed as follows, fig. 27 :

Fig.27

MW = trace of pieture plane in the principal plane, ER = trace of horizon plane in the principal plane, GG — trace of ground plane in the principal plane, 880 = élévation of the station 8 above the ground plane GG, or above the foot of the station 80.

A scale of equal parts is laid off upon GG, to either side of P0, and radiais are drawn from 8 through the graduation points of the scale ; their intersections with MW form the vanishing scale, which may serve to locate distances from the foot of the station to points to be platted in the ground plane.

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