Levels manufactured from fine iron castings with a involute 'V' groove between the bearing flats for accurate seating on round work. Ground glass main and cross test vials. The 310mm version in addition has a plumb vial fitted.,The main vial is adjustable to positive setting and marked with graduations, 2.5mm apart, to BS3509 sensitivity. A bubble movement of 2.5mm (1 graduation) indicates a tilt from the horizontal of 0.12mm per 300mm.