Architectonic Sector, by George Adams, London, c. 1760

An example of the architectonic sector as published by Joshua Kirby in The Description and Use of a New Instrument called an Architectonic Sector ... (London, 1761). The instrument is incomplete, with only its sectoral legs surviving; the arc is missing (along with the screws to hold it in place). But there is no doubt as to its identity: there are slots to receive the arc and bevelled edges on each face marked to identify the orders (TDICR) and numbered to match the corresponding scale line on the arc (1-22; 23-43). Each leg carries a scale of equal parts, variously divided [0]-XV and [0]-60, subdivided to 10, 5, 1, third, sixth.

The dating comes not only from Kirby's book but from Adams' signature: the Prince of Wales became George III in 1761.

For a separately surviving arc, see inv. 40175.