Note 57 — Chapter V · (continued 2)

Note 57.—On the Astrolabe. The astrolabe is more commonly used by the Arabs than any other instrument for astronomical observations. It is generally between four and six inches in diameter. It consists of a circular plate with a graduated rim, within which fit several thinner plates, and of a limb, moving on a pivot in the centre, with two sights. The plates are engraved with complicated diagrams, &c., for various calculations. The instrument is held by a ring, or by a loop of cord attached to the ring, during an observation; and thus its own weight answers the same purpose as the plumb-line of the quadrant (which the Arabs sometimes use in its stead); the position of the moveable limb with the sights marking the required altitude.