Polygon of Forces
“In the apparatus shown... three forces act on a small body and are allowed to assume a position of equilibrium. A triangle is then constructed with sides parallel respectively to the three forces; it will be found by measurement that the sides are also the lengths proportional to the forces, and as will be seen the arrowheads point concurrently round the triangle.
Benson John Lossing, ed. The New Popular Educator (London, England: Cassell & Company Limited, 1891)