A rectangular tube to one side of which is fastened a vertical, graduated arc with vernier and carrying a rotatable level. The instrument is small and light enough to be held in the hand. When used as a levelling instrument, the vernier is set at zero on the graduated arc. When used to determine vertical angles, the object is sighted through the tube and the vernier rotated until the bubble is centred in the vial. The angle is then read off the graduated arc. This levelling instrument is named after W. deW. Abney, an English physicist and, with Locke's level, is one of the best known of its type.