14. Movable quadrant by J. Sisson
London, 1739
Jonathan Sisson (1690?-1747)
iron, mahogany wood and brass
1 telescope
radius 107 cm
[Inv. MdS-120]

The handling of this instrument - whose purpose was essentially to determine when exactly a star had passed over the meridian so as to be able to work out what was south and adjust meridians and mural instruments - was done by entirely mechanical means, in contrast to the instruments of the late XVIIth and early XVIIIth centuries. The astronomer thus required a good deal less manual ability for setting up the instruments and executing observations. In this field too, therefore, technology was beginning to build functions into machines that were once man’s competence. This was a characteristic trend of the XVIIIth century that was paving the way for the move from artisanal to industrial production.

"Movable quadrant by the Englishman Gionata Sisson of 2.9 foot radius, with fixed telescope, and all metal perpendiculum, supported by robust mahogany frame finished in brass", we read in the inventory of 1746 (Arch. Stato Bo., fondo Assunteria d’Istituto, Div. 11). "The limb of the quadrant has very accurate divisions which, being laid out in a Tychonic scale, can give the 1/3 arcmin or 20 second value for the direction of the plumb-line.
Thanks to 4 screws at the base of the device and an air-bubble level attached to the vertical axis that supports the Quadrant, it is possible to make the said axis perfectly vertical, from which with the right device the limb of the quadrant can be brought into perfect contact with the plumb-line. Four metal plates lie under the screws of the base. A brass handle turns the above-mentioned screws.
The pendulum is missing; the cross-piece between the base and the quadrant - which transmits the fine variations in inclination - has been restored (G. Morigi, Bologna, 1979), while the relative screw is original.
Written on the limb is J. Sisson/London, undated; on the lens of the transit telescope by the same author and ordered at the same time, however, is the date 1739 which allows us to date this instrument.
The precision and detail - both technical and artistic - of this instrument and of the other works by Sisson in the Specola, such as the transit telescope and the mural quadrant [files 17 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.119 and 107, fig. 18.
M. Daumas (1953).
D. Diderot, J.B. le Rond d’Alembert (1750).
D. Howse (1993), p.608.