Minding has proved a remarkable theorem concerning a variable system of forces defined as follows, the points of application of the different forces, and their magnitudes are given, while the directions are such that a pencil of rays through any given point parallel to them moves as a rigid body.

Besides Minding's original investigation, several others have been given since. The last of these, due to Professor Tait, rests on purely quaternion methods, and is so elegant and concise that I was led to reinvestigate the whole subject by ordinary methods in the hope that the analysis might have some points of interest.