Rotating furnace
![Axially symmetrical paraboloid. The inside surface is concave](/Images/godic/202502/08/Paraboloid_of_Revolution.svg2643.png")
![Parabolic shape formed by a liquid surface under rotation. Two liquids of different densities completely fill a narrow space between two sheets of plexiglass. The gap between the sheets is closed at the bottom, sides and top. The whole assembly is rotating around a vertical axis passing through the centre](/Images/godic/202502/08/Parabola_shape_in_rotating_layers_of_fluid2643.jpg")
A rotating furnace is a device for making solid objects which have concave surfaces that are segments of axially symmetrical paraboloids. Usually, the objects are made of glass. The furnace makes use of the fact, which was known already to Newton, that the centrifugal-force-induced shape of the top surface of a spinning liquid is a concave paraboloid, identical to the shape of a reflecting telescope's primary focusing mirror.