17. Mural quadrant by J. Sisson
London, 1739
Jonathan Sisson (1690?-1747)
brass
radius 172 cm
[Inv. MdS-121]

Jonathan Sisson was the best pupil of Graham, the maker of the instruments ordered by Halley for the Greenwich observatory. His instruments were famous for their detailed perfection, all the more incredible if we bear in mind that in those days there were no machine-tools and all the work was completely manual. This quadrant is the oldest, made entirely from a single metal which protected it from the distorsions that variations in temperature caused instruments made of brass and iron.
An accurate description of the instrument and its accessories is to be found in the 1843 Inventory by Gaetano Ceschi:

"Mural quadrant by the Englishman Jonathan Sisson with 3 and ˝-foot radius, all in brass, fitted with telescope with object lens by Bruni of 1 and ˝-inch aperture, and eyepiece with a string micrometer. To the Telescope is attached a Nonius, which slides with it along the graduated limb of the quadrant, and which serves to give the fourth part of an arcminute, or 15 seconds.
Two metal supports fixed to the wall by screws carry the whole machine. (not found)
Four small metal clamps attached to fittings built in the wall serve to keep the machine straight along the meridian. (not found)
Brass frame which is placed over the eyepiece and which is used to carry a colored glass plate with metal frame. (not found)
A small reflector with brass shank which goes on the object lens. (not found)
A brass ball with small hook which is attached to the end of a metal wire and which serves as perpendiculum for rectification. (not found)
A small walnut shelf hung on wall, used to carry a glass with water in which is dipped the ball of the perpendiculum. (not found)
N. 3 brass handles for adjusting the support screws of this machine. (not found)
The Telescope is counterbalanced by a hard sandstone counterweight placed at the end of a lever arm, which via an iron axis remains attached to the mahogany frame, that is connected to the telescope. (not found)
Two metal supports fastened to the top of the wall by screws, between which revolves the axis of the lever arm which supports the counterbalance. (not found)
In the opposite wall 2 other metal supports are fastened with screws, together with the 4 clamp supports for carrying the quadrant when it is to be used towards the North. (not found)"
Replaced in 1979 on the east side of the reconstructed meridian wall, the instrument bears on its limb the words Jonathan Sisson London. The 1746 inventory states that the counterweight was made of lead and not "sandstone". The long wooden square parallel to the telescope is original; the attachment to the telescope, lead counterweight and its mounting have been restored (G. Morigi, Bologna, 1979) as have the other parts fixed to the wall. The micrometric screw and clamp attachment to the limb of the quadrant have also been restored (1979).
The date when the original object lens was replaced by that of Bruni is not known. On the object lens of the transit telescope by the same London craftsman, ordered at the same time, we find the date 1739.
A handwritten note by Eustachio Zanotti in the Reg. Sp. Ist. Scienze, vol. VII, dated 16 August 1742 (Arch. Dip. Astron. Bo.) reveals that the instrument became part of the Specola’s equipment on that date. Finally, in 1873, it was removed from service.

The precision and detail - both technical and artistic - of this instrument and of other works by Sisson, such as the transit telescope and the movable quadrant [files 14 and 19] were such as to earn them a place, as models of astronomical equipment, in the tables of the Encyclopédie by Diderot and d’Alembert.

E. Baiada, A. Braccesi (1983), pp.117-118, fig. 16.
D. Diderot, J.B. le Rond d’Alembert (1750).
D. Howse (1993), p.608.