Astronomical Museum - Catalogue of the instruments
Details of the instrument
- Name: Mural quadrant
- Producer: Ramsden
- Place of production: Londra
- Year of production: 1789
- Collocation: MNST
- Institution: MNST
- Stock Number: 1025
- File: The astronomers of Brera soon realised that Canivet's quadrant was insufficiently precise in the measurements of position. The analyses of 1773-1774 had brought to light excessive errors linked to the division of the border. The astronomers therefore began to demand a larger mural quadrant, with a radius of 8 English feet (2.44 m), produced with greater care.
From 1774 onwards, every occasion was exploited to obtain the funds necessary for the purchase, and at last the right moment came. In 1785, Oriani prepared for his journey to London in the following year, and asked for the funds required for a quadrant, which Oriani himself was to order there. Its construction specifications were to be defined in greater detail in the direct contacts between Oriani and the manufacturer. In the projects of the astronomers, the new quadrant was to be used in the theory of the planets and the precise determination of the positions of the stars. However, taking advantage of the government's interest in this subject, the official request also added that the positions of the stars would also be useful for the precise location of certain areas of Austrian Lombardy on the occasion of the production of a new map of the region. Boscovich also put pressure on the governor to grant the request.
The authorisation for the purchase was received in April 1786, making explicit reference to the use of the quadrant for the geodesic operations.
The construction of the quadrant, commissioned from Ramsden, began in 1789. In that year, the astronomers informed the government of the news and asked for the balance for the payment (500 gold pieces had already been paid when the order was placed). At the end of 1790, however, the quadrant was still in London, due to difficulties in its delivery. The cases were larger than usual and a ship with sufficiently wide hatches had to be found, to avoid widening and then repairing those already in existence. In addition, the hire costs were higher than usual, because the cases had to be stowed in an isolated position in the hold, without others stacked on top of them, to avoid damage, and the useful space not occupied by other goods had to be paid for. Perhaps the official business representative in London who was responsible for the despatch of the instrument, Antonio Songa, could have acted in a more decisive manner.
Finally, at the start of December 1790, the quadrant left London, and arrived at the port of Genoa in mid-January 1791. After a brief delay, it reached Milan, probably in March. On 1st June, the instrument was already installed in the quadrant room, aimed towards the south, in the place of Canivet's quadrant. In 1792, the astronomers completed the position checks, and the quadrant remained in use until around 1840.
The quadrant has the typical structure of Bird's instruments. It is manufactured completely in brass and the structure is solid and well made. Unfortunately, various parts of the instrument are missing, with only the structure, border, telescope tube and vernier remaining. There are two series of divisions on the vernier. In the inner part, we can see the division in degrees, with minimum intervals of 5' and numerical indications every degree and every 15'. On the outside, we have the division in 96 parts, each of which is divided into 16 intervals (the numerical indications are at each main subdivision and every 4 fine divisions). At the free end of the telescope, there is the plate with the two verniers and the blockage shoe with the micrometric screw. The vernier with the scale of degrees is divided into 10 parts and 30" can be read. The vernier of the other scale is divided into 16 parts and minimum readings in the order of 13" are possible. The reading of the single seconds was obtained with the screw micrometer, which the astronomers considered highly regular and precise. The divisions continue beyond zero for 6° and 6 parts of the second scale.
The telescope tube, which is also in brass, was fixed at five points to a rhomboid shaped support structure to avoid bending.
The objective was an achromatic duplet lens system with a diameter of roughly 9 cm (36 lines) and a focus length of 244 cm (8 feet) (all the eyesights are now missing).
The focus contained the reticle, which was initially formed by silver wires, five vertical and one horizontal. The intersection of these with the central vertical wire determined the line of confidence. The reticle could be moved and adjusted in the vertical, horizontal and oblique directions. In 1817, De Cesaris replaced the silver wires with copper ones, which were thinner and more precise, though more delicate. To prevent the telescope from bending, there was a system of counterweights, which apparently was particularly effective. As well as a main counterweight, the best quadrants also had a second weight fitted to the arm at right angles to the first. This was necessary because the force exerted on the telescope by the counterweight varied as the inclination altered. The second counterweight compensated for the reduced action of the first when the telescope was inclined upwards.
In the Milan quadrant, Ramsden added a third counterweight for the first time. This had a constant position and force, and was applied to an arm that acted on the rotation axis, to offer it support.
The position of the quadrant in the meridian plane was checked by comparison with observations of stars at various heights carried out by the transit telescope. For this purpose, the formulae calculated by Boscovich were used, as these were able to assess the errors in a known point and those in three points. The deviations, however, were minor throughout the extension of the border.
The first tests on the machine were carried out using mechanical methods. The vertical level of the side corresponding to zero was initially determined with the plumb line of the instrument (which now, like all the other accessories, is missing). The horizontal level of the 90° side was checked using a device that had recently been invented by Ramsden, the "wire plumb line".
Later, more precise analyses were carried out. Oriani, observing Mercury, checked the plane of the quadrant to compare it with measurements obtained with Canivet's sextant using the corresponding height method. During the observations, he noted that the differences between the values read on the two scales rarely exceeded 1". Comparison with Canivet's sextant was also used to establish the zero point of enumeration, with observations on the position of Alpha Auriga (Capella), a star that, at the latitude of Milan, passes very close to the zenith, and for this reason often used as a point of reference by the astronomers of Brera. Much later, in 1833, a careful check of the zero point of enumeration was carried out. Together with the mural quadrant, Carlo Kreil also used a Kater collimator, purchased to establish the zero point of enumeration of Starke's meridian circle. The observations, made between September 1833 and January 1834, consisted of five measurements, repeated every 3 hours, between six in the morning and midnight.
Kreil intended to reveal the sources of error, separating the construction and positioning errors from those linked to the movements of the entire structure and the temperature variations. He therefore placed a first thermometer outside the supporting wall, with the bulb covered in white paper to avoid direct sunlight, while a second was suspended near the quadrant. Kreil, however, was unable to obtain the results he expected, that is, a particularly precise table of corrections that would have enabled him to calculate precision measurements with the quadrant that could be compared with the instruments of more recent construction.
- References: State Archive inventory;
Historic Archive of the Observatory of Brera, New Administrative Archive, file 41, sheet 143: F Carlini: "Notizia della prima fondazione della Specola e de' suoi successivi incrementi", 31 August 1841;
Historic Archive of the Observatory of Brera, Old Administrative Archive, file 9, sheet 18, April 1786;
Historic Archive of the Observatory of Brera, Scientific Correspondence 1790-1792, file 87: 10th September, 30th September, 26th November, 3rd December, 10th December 1790;
Angelo De Cesaris, "De quadrante murale", Ephemerides Astronomicae Anni 1792, Appendix, 1791, 18: 73-104;
Angelo De Cesaris, "Osservazioni del Sole al quadrante murale", Effemeridi astronomiche di Milano per l'anno bisestile 1804, Appendice, 1803, 30: 46-72;
Angelo De Cesaris, "Sul movimento oscillatorio e periodico delle fabbriche", Effemeridi astronomiche di Milano per l'anno1813, Appendice, 1812, 39: 105-116;
Angelo De Cesaris, "Riflessioni pratiche sulla misura del diametro del Sole", Effemeridi astronomiche di Milano per l'anno 1819, Appendice, 1818, 45: 3-11;
Carlo Kreil, "Osservazioni al collimatore di Kater applicato al quadrante murale di Ramsden", Effemeridi astronomiche di Milano per l'anno 1835, Appendice, 1834, 61: 130-138.