Conoids, string surface model, 1872.
© Science Museum / Science & Society Picture Library
Model showing the transformation of a cylinder into a conoid and back again. The transformation is effected by making each of the two upper semicircles turn through a right angle. It is one of a large set of ruled surface models made by Fabrice de Lagrange following designs introduced in the early 19th century. They illustrate surfaces which can be traced out in space by the movement of a straight line.