This sizeable instrument was designed to be used in King George III’s private astronomical observatory in Richmond. Designed by Jeremiah Sisson of The Strand in London in 1770, it’s still owned today by the Crown Estate Commissioners. It’s nearly three metres in height and over four metres wide and it was used to make astronomical measurements. Sisson might have been a fine maker of instruments, a skill he had inherited from his father Jonathan Sisson, but he wasn’t as successful financially.
The museum’s blog mentions that one of the challenges was moving it from storage into the museum itself, as it weighs around 450kgs and needed a sling around it when it was being hoisted into place. It was also restored so that it looked like new and Sisson’s signature could once again be seen, although personally I prefer to see these things in their dilapidated state as it feels that bit more authentic.
There are some old books which give lots of detail on how the mural arc was originally used, but it’s all a bit complex for me to understand. It’s an impressive piece of apparatus though, with an interesting heritage.