Mathematical instruments in the writings and in the hand of practitioners: geometric square, from Gerbert to Gall
The geometric square (quadratum geometricum), a portable instrument in the form of a square frame for ascertaining distances and heights by measuring angles, was first presented by Gerbert d'Aurillac (Pope Sylvester II, 946--1003) in his \textit{Geometrica}. Since that time, many European mathematicians discussed the properties of this instrument, and later there appeared producers of such a useful device. The present author elucidates the question, giving special attention to the contribution made by Portuguese priest Chrisostomus Gall S.J. (1586--1643).
scientific article; zbMATH DE number 6183038