or Gunner's Callipers, is an instrument in which a right line is so divided that the first part being equal to the diameter of an iron or leaden ball of one pound weight, the other parts are to the first as the diameters of balls of two, three, four, or more pounds are to the diameter of a ball of one pound. The caliber is used by engineers, from the weight of the ball given to determine its diameter or caliber, or vice versa.

The gunner's callipers consist of two thin plates of brass joined by a rivet, so as to move quite round each other. Its length from the centre of the joint is between six inches and a foot, and its breadth from one to two inches; that of the most convenient size is about nine inches long. Many scales, tables, and proportions, &c. may be introduced on this instrument; but none is essential to it except those for taking the caliber of shot and cannon, and for measuring the magnitude of salient and re-entering angles.