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

731

The eenter of s is placed directly upon the point that marks the platted caméra station on the ground plan. The ruler BT is placed parallel to the ground line of the picture plane, and s and BT are now secured in this position on the ground plan.

When the arm sM is moved, s being held in a fixed position (to coincide with the platted station point), the point O will follow the motions of the arm sM, also applying its motion directly to the arm OD (which slides in the groove of XY) and indirectly to the arms of the pantograph System.

The fourth sliding piece {/Y), being connected with the joint A of the pantograph System by means of a separate piece, insures a permanently fixed distance between (yu) and A while the instrument is in use.

The pantograph System is composed of six pièces: Four straight arms, AB, AG, F/j, and F/Y and two double arms or levers, GDF and BD G, which are bent at right angles at their points of junction D. The sides of the two parallelograms ABGD and DGFE are ail of equal lengths, and the six arms are joined in A, B, G, D, F, F, and G. The arms F/j. and F/j' are twice as long as the length of the side of the parallelograms.

The pencil which describes the perspective may be attached to the free end of either arm F/j or F/j‘ .

The angles GDB and FDG being each equal to 90°, the sum of the two other angles GDB

and GDF must be equal to 180°, and as the sum of two adjacent angles in a parallelogram is equal to 180°, it follows that

GDB + GDF = GDB + DGA or:

GDF = DGA

This shows that the two parallelograms FGDF and GDB A must be equiangular, and as their sides are equal in length, the parallelograms themselves must be equal, and the diagonals FD and GE of the one are equal to B G and AD of the other, respectively.

The two long arms F/j' and F/j being of equal lengths, /j/Y will be parallel to GF, both will be perpendicular to the direction of XY, and /j/j' will pass through D. We hâve, therefore, D/Y = D/j = GF = DA.

Use of the perspectograph.—The sliding piece s is secured to the working board, over the platted position of the caméra station on the ground plan, still permitting a gliding movement of the arm sM in the direction sM (fig. 124). The eenter line of BT is brought into a position parallel to the platted ground line, and its position is also secured to the board. The sliding piece (S), ünally, is moved from s (in the groove of BT) until s (S) is equal to the élévation of the station S above the ground plane, also securing (S) in this position, when it will still permit a gliding movement of the arm (8) M in the direction of (S) M. The eenter line of the wooden ruler XY is placed upon the ground line (picture trace) on the ground plan.

The manipulation of the instrument and its general working will now readily be understood.

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