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UNITED STATES COAST AND GEODETIC SURVEY.

(3-,)

For instance, when the tracer M is moved in a direction parallel to BT or to XY, tlie arm sM will move the slide OD in the same direction. The distance O (pi) remaining unclianged—as long as s (8) undergoes no change—(/a) A will also remain of a constant length. Hence, AD and also GI1, as well as D/a, undergo no changes, and the pencil in ja or in /a' will trace a line parallel to XY, representing the perspective of a line of the ground plan (the one traced by M) parallel to the

picture plane.

When M is moved in the direction of sM, away from XY, the positions of O and D remain the same, but O/a will be lengthened, (/a) moves to the right, or away from 0, carrying the point A with it (A (/a) being a constant length) and increasing the length of the diagonal DA in proportion to the increase of the length of 0 (pi). DA being eqnal to GEJ = Dpi (Dpi1), the latter will also be lengthened, and /a will be moved down, or away from XY, by the same amount as (ja) is moved to the right. The relation between the construction made in fig. 123 and the mechanical platting by means of the perspecto-grapb will now be évident.

VII. Prof essor HaucWs trikolograph.—This instrument has been described by Dr. G. Hauck in a memorial commemorating the opening of the new building of the Boyal Teehnical High School at Charlot-tenburg, near Berlin, hTovember 2, 1884. It serves to reconstruct an object from two perspectives of the same that had been obtained from two different points of view.

The principles which underlie the construction of this instrument hold equally good for the construction of an instrument to be used for the mechanical platting of the ground plan of any object represented on two photographs obtained from different stations.

In 1887, Prof. F. Schiffner already suggested the changes to be made to Dr. Hauck’s instrument, in order to render it available as an instrument of précision for the use of the phototopographer ; still it seems that mechanical diffi-culties in its manufacture are yet to j[a.)

be overcome, as the writer has not met with any record of such a per-fected instrument havingbeen either in use or even been constructed.

In Chapter I it had been shown that a point, A, photographed from two stations S and Sx, may be platted in horizontal plan, if the two picture traces, gg and gxgx, and the two caméra stations, S and Sx, are given on the horizontal plan, fig. 125.

The two picture planes may be revolved about their ground lines, gg and gxgx, into the ground or platting plane, when (a) and (af) will be the two images of the point, A, revolved into the ground plane. If we draw lines through (a) and (ax) perpendicular to the corre-sjmnding ground lines gg and gxgx, then a' and a/ will be the (horizontal) projections of the picture points, a and ax, into the platting plane, and the intersection, A', of the radiais Sa' and Sxax' will locate the positions on the platting sheet of the point A, pictured on the two plates as a and aH respectively.

This graphie détermination of the platted position A' of the point A may be accomplished mechanically by placing slotted rulers with their center lines upon gg and gxgx, fig. 126, and indicating the directions of the perpendiculars, dropped from the pictiired points (revolved into the horizontal plan) upon the ground lines, by two arms (a) be and a'b of a pantograph combination, where

(a)b = bc — a'b

or '

(ax)bx — bxCi = a'ifti.

Fig. 126

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