|These are the partial and badly damaged remains of a transit instrument by John Bird. Despite the name, a “transit instrument” usually has nothing to do with the transits of Venus. Its principal purpose is to observe the Sun or stars as they cross (transit) the north-south meridian line.
This particular instrument is from Thomas Hornsbys private observatory, set up in his Oxford home. He used it to regulate the astronomical clocks responsible for timing the 1769 transit of Venus.