Note 10 — Chapter XXXIII
Henry Yule translation
The Alidada is the traversing index bar which carries the dioptra, pinnules, or sight-vanes. The word is found in some older English Dictionaries, and in France and Italy is still applied to the traversing index of a plane table or of a sextant. Littré derives it from (Ar.) ’adád, enumeration; but it is really from a quite different word, al-iḍádat عضادة “a door-post,” which is found in this sense in an Arabic treatise on the Astrolabe. (See Dozy and Engelmann, p. 140.)