The ornamental design for a weighinginstrument, as shown and described.
FIG. 1 is a front perspective view of the weighing instrument, with broken lining on a surface showing functional elements that are not considered part of the design;
FIG. 2 is a left side elevation view of the FIG. 1 device;
FIG. 3 is a right side elevation view of the FIG. 1 device;
FIG. 4 is a front elevation view of the FIG. 1 device;
FIG. 5 is a rear elevation view of the FIG. 1 device;
FIG. 6 is a top plan view of the FIG. 1 device; and,
FIG. 7 is a bottom plan view of the FIG. 1 device.