Engraving of the Great Equatorial Telescope of the Paris Observatory. This view shows an equatorially mounted refracting telescope of 31 cm (12.8 inch) aperture made by the French maker Lerebours. The telescope was installed in 1860 within the western dome of the main building at the Paris Observatory. An equatorial mount is a mount for instruments that follows the rotation of the sky (celestial sphere) by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any object in the sky that has a diurnal motion by driving one axis at a constant speed. Coloured illustration from 'The Heavens' by Amedee Guillemin, Publ. Richard Bentley, 1878.