3. Apparent and mean solar time meridian and string meridian by E. Lelli
Bologna, 1741-1742
Ercole Lelli (Bologna 1702 - 1766) brass and marble
length 635 cm
[Inv. MdS-109]
Lamp of meridian in brass, height of meridian line 245 cm
[Inv. MdS-110]
String supports of meridian in brass
[Inv. MdS-111]
Fittings of meridian string in brass
[Inv. MdS-112]
Size of Meridian Room 719 cm x 444 cm x 267 cm

The first exemplar of a string meridian that we have a full description of was made in 1713 in the Observatory of the Tower of Luxembourg in Paris by Joseph-Nicolas Delisle (1688-1768) (op.cit.). However, in the 1703 inventory of the instruments in the Marsili Observatory, we find written, in Manfredi’s hand, Meridianae filares quattuor pro nunc, an entry which suggests that this type of use of the most common meridian line was already widespread before Delisle’s written work.
It was a true astronomical instrument, unlike the apparent and mean solar time meridian, whose purpose was often purely ornamental.
Even if they provided greater accuracy in computing the transit times of celestial objects across the meridian, the mural instruments were, in fact, not always able to guarantee the planarity of the graduated limb. It was therefore preferable, at least in the case of the Sun, to calculate the moment of its transit across the meridian with a different instrument whose main part consisted of a simple string stretched between two fixtures, which, by its very nature, could not deviate from a straight line. Once the string was perfectly oriented along the meridian, the transit time of the Sun was no longer affected by the different elevations on the horizon it has in the various seasons of the year. The moment of transit could likewise be computed with an accuracy of about half a second of time, accurate enough if compared to the operative regularity of the clocks of the period.
The differences between the transit times of the Sun measured with the meridian line and those measured with the telescopes of mural instruments made it possible therefore to calibrate the non planarity of their limbs and correct the transit times computed for the other stars.
Things went on like this until the introduction of transit instruments, but the string meridian remained for the whole of the XVIIIth century the standard instrument for the determination of noon, both for its simplicity and its stability.

It was therefore indispensable for Manfredi to set up a room, inside the Specola, that would consent observation of the meridian transit of celestial objects and allow the building of a string meridian that was accurate enough.

"Likewise a shutter with its fitting for maneuvering it was made and fitted with the aim of covering and uncovering the hole that lets in the Sun on the meridian in the said Room of the Semicircle and two notches were made in the two iron fixtures placed at the two ends of the said meridian through which the string marking the arrival and exit of the sunlight is stretched in the plane of the meridian." (Manfredi in Reg. Sp. Ist. Scienze Bo., Vol I, dated December 1726).
There are many notes preserved in the Archives of the Department of Astronomy regarding both its use and restructuring, which occured in 1741 with a view to the imminent arrival of the English instruments by Sisson [files 14, 17, 19, and 35].
The Commentari of the Academy of Science (T.II, part I, 1745, p.40) tell us that the builder of the new meridian line - still visible as indeed is the Tuscan order decoration with columns, traces of which can still be found on the walls of the room - was the Bolognese Ercole Lelli, history and portrait painter, scenographer, architect, mechanic, sculptor and anatomist.
On that occasion the wall which supported Lusverg’s great mural semicircle was modified and support columns for Sisson’s transit telescope erected [file 19]. The old wooden floor was replaced by today’s.
Of the original 1726 structure, the perimetrical walls remain along with the two strong support arches built on the structures of the first floor of the building and inclined according to the diagonals of the room.
The two meridians are described in Ceschi’s 1843 inventory as follows:
"Apparent solar time meridian built in the floor of the apposite Room (Meridian) with brass foil edged round by 2 strips of marble, with Zodiac signs around in brass, encased in marble squares. Mean time meridian of brass foil built round the afore-mentioned. A brass lamp fastened to the wall of the room, in which in silver plate the hole is made that serves as gnomon for the said meridians. At the ends of the first meridian along the lateral walls on the floor there are two specially designed brass pieces which serve as support for the string, that is stretched above the same, in order to determine the arrival and exit of the solar image, and estimate the mean time. These supports for the string can be corrected as appropriate.
A small board with stretched paper which is placed under the string where the solar image is projected."
The lamp was designed to allow the fall of a plumb line from the opening through which the sunlight enters and strikes the string of the meridian. The string could be adjusted to just touch the plumb-line. The position of the string on the support was governed by the V-shape and two small weights attached to the ends, "two brass balls", served "to stretch the string".
The north wall still carries the three "brass nails for use of the said string", i.e. which were used to pick up the string of the meridian.
In 1815 the transit telescope of Sisson was replaced by a more modern one by Reichenbach, Utzschneider und Liebherr [file 20] and the original support columns were replaced by others, placed closer together and of Ionic order.
In 1912 the mural quadrant of Sisson and the transit telescope were taken out of service, the support wall of the quadrant demolished and the columns knocked down. The roof, which originally opened along an east-west axis, was replaced by a normal covering. Finally, in 1952, with the construction of the aula magna of the Institute of Astronomy, whose outer wall fits into that of the Meridian Room, this latter was reorganized again and the roof raised. The room was used as a deposit for books.

The restoration work carried out in 1979 by the architects Mauro Monesi and Luigi Suffritti could not be complete. The opening of the cover was not restored but its original existence was underlined by leaving a large 89 cm hole which according to the documents was the size of the original opening.
The meridian wall was rebuilt slightly longer than the 1741 version - which was about 293 cm vs x 31 cm thick - to carry the restored Lusverg semicircle [file 16].
In 1990, in order to verify whether there were still traces of the old wooden floor - used by Manfredi to set up some of the instruments and which we know was six feet, one and a half inches (about 264 cm) from the rotational centre of the semicircle’s telescope - a cut was made in the floor, under the window door that opens onto the terrace to the south-west, but no sign of any preexisting floors was found.

E. Baiada, A. Braccesi (1983) p. 89 e 11.
J.N. Delisle (1713) p. 5.
B.M. Oliver (1972) p. 20.