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

735

This change in the position of (g) produces a corresponding change in the sliding point (<?i), which in turn changes the position of the tracer (a{), causing the pantograph arins (cti) Ci and bxa\ to move, and a change in the position of will cause the radial ruler RiSi to assume a new position also. The intersection of ES with the new position of RiSi will locate the platted position in horizontal plan of the point under the tracer (a) on the first photograph, without having actually identified the corresponding image of the (same) point under the tracer (ai) on the second picture.

If a line on either photograph is followed out by one of the tracers (a) or (ai), the pencil point A' will draw the horizontal projection of the line given in perspective (the second tracer being observed ehiefly as a check or to aid the general working of the instrument by a gentle tappiiig when the movements of the various parts of the instrument are retarded by too much friction or lost motion).

Until now no perfect perspectograph has been constructed, and, no matter how accurately such instruments, like the one just described, may be made by the mechanician, there will always remain some unavoidable imperfections in the material or in the workmanship of the instrument that will produce more or less error in the results.

For accurate and précisé work, therefore, the iconometric platting should be accomplished with the aid of graphie or geometrical constructions for ail the control work of the survey, using perspective instruments only for filling in such details which, in an instrumental survey of a similar character, would be sketched in by the topographer.

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