Conoids; model showing the transformation of a cylinder into a conoid and back again, also model showing the transformation of a cone into a conoid and back again. It is to be noticed that the head lines of the two conoids, that is to say, the right line in which the two sheets of each conoid meet, are perpendicular to one another; - the transformation is effected by making the upper semicircle turn through two right angles

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 Fabre de Lagrange in Paris, France, 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.