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an instrument for measuring a vertical distance between two points, which need not be situated along the same vertical. A cathetometer consists of a rod mounted vertically with the aid of a level and three leveling screws; a horizontally placed telescopic tube, which can be moved along the rod while remaining perpendicular to it; and a device for the accurate aiming of the tube. The eyepiece of the cathetometertube is equipped with crosshairs. In using a cathetometer thecrosshairs are successively aimed at each of the selected points, and the unknown distance is determined by the displacementshown on a scale on the cathetometer’s rod. The cathetometerwas invented by the French physicists P. Dulong and A. Petit(1816); various improvements in its construction were con-tributed by D. I. Mendeleev.