The weathervane works fundamentally in the usual way: the pointer is turned by the wind. However, rather than turning freely on the rod to which it is attached, it actually turns the rod as it moves. That allows the information to be transmitted to the pointer. The instrument was made locally: embossed on its face are the words Bennett, Greenwich, and Blackheath.The Bennetts were a clockmaking family in Greenwich.
More conventional in function, but less so in form, is the beaver-shaped weathervane on 60 Bishopsgate. It's a reminder of the building's former occupant, the Hudson Bay Company. Established in 1670, its founders were actually French but won the patronage of Charles II, who granted a charter to his cousin Prince Rupert. The company was given the lands around the Hudson Bay, and only ceded ownership of this territory to Canada in 1870. From that date, it focused upon retail business and now has department stores across Canada. After three centuries in London, it moved its headquarters to Toronto in 1970, leaving this distinctive weathervane as a reminder of its presence in the City.
Nearby, the weathervane on St Ethelburga's (a former church, now a Centre for Reconciliation and Peace) is dated 1671. That may suggest rebuilding after the Great Fire of 1666, the fate of most City churches, but in fact this building survived the fire and dates back to the fifteenth century. It did, however, suffer serious damage in the IRA's 1993 bombing of Bishopsgate, following which it required significant restoration. Among the surviving pieces found in the rubble was this weathervane.
I've previously looked at the Whitechapel Art Gallery weathervane, the Viking longboat catching the breeze on St Olav's Norwegian church, and the Royal London Building on Old Kent Road.