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A Theodolite and Sundial Attributable to Gaulterus Arsenius

Spring, 1998

RECENTLY rediscovered in the Museum was a hitherto unknown and unrecorded instrument, unsigned and undated, but confidently attributable to Gualterus Arsenius, the illustrious Flemish instrument maker active in Louvain between 1554 and 1579.

The instrument is a square simple theodolite combined with a sundial for equal hours, made for use between the latitudes 30 and 63 degrees. The sides are 364 mm in length, which is large when compared with contemporary instruments of the same type. It is in relatively good condition: the 400 years that separate us from its construction have certainly not damaged the engraving in any significant way, and it is partly as a result of this that an attribution is possible.

The instrument consists of two parts: a square base-plate, with three circular scales on the outside enclosing a wind rose marked with thirty-two wind directions, seven between each of the four cardinal points. The names of the winds are given in Dutch. On the four corners are engraved in turn the large capital letters ‘A’,’B’,’C’ and ‘D’.

A circular plate lies above the base-plate and is free to rotate about its centre. The outer edge of this plate touches the inside of the second circular scale engraved on the base-plate, and has four pointers, one every 90 degrees. The front of the circular plate also has a compass at its centre, used to orient the instrument when used either as a theodolite or as a sundial. Both the compass needle and glass cover are missing.

The surface of the top plate is fully engraved with lines for the sundial. Circles, centred in the middle of the plate, indicate the different latitudes between 30 and 63 degrees, while arcs radiating from the centre correspond to the hours (with dashed lines for whole hours, full lines for the half hours). One half of the plate functions for the hours in the morning (IIII to XII), while the other half is meant for reading the hours after midday (I to VIII). One quarter of the plate (where the night hours would otherwise be) is partly cut away, leaving an opening through which the names of the winds on the plate beneath can be read.

On the edge of the circular plate at the six o’clock point in both the morning and evening are traces of where sights, now lost, must have originally been attached to the instrument. These sights would have been necessary for using the instrument for surveying. In use, the instrument would have been arranged horizontally, and on the back there is a socket into which the shank of a staff or the head of a tripod can be inserted in order to achieve this. Once horizontal, distant objects would be sighted though the vanes or pin-holes of the lost sights by rotating the upper plate, with bearings read off in degrees from the circular scale or as directions from the wind rose.

In order to use the instrument as a sundial, some form of gnomon would be needed to cast a shadow over the hour lines. The original gnomon for the instrument is, like the other detachable parts, missing. It is not certain exactly what form the gnomon would have taken, but one possibility is that it was a string gnomon, whose angle would have been adjusted for the latitude where the sundial was to be used. The string could have extended from the centre of the compass to a raised support fitted into the hole at the corner marked ‘C’.

An almost identical instrument to the Museum’s theodolite-sundial, though at 291 mm square somewhat smaller in size, is preserved in the Württembergisches Landesmuseum in Stuttgart. Unlike the Museum’s instrument, the Stuttgart example is signed and dated – ‘Nepotes Gemmæ FrisY LouanY fecerunt anno 1579’. The function and scale divisions of this instrument are exactly the same as the Museum’s, apart from the fact that the sundial is laid out for 9 degrees less latitude: from 35 to 59 degrees only.

The signature on the Stuttgart instrument deserves special attention, since the word ‘nepos’ appears there in its plural form ‘nepotes’.

In twenty-seven of his surviving instruments, Gualterus Arsenius signs himself as the ‘nepos’ of Gemma Frisius, the famous Louvain mathematician, physician and author. The term ‘nepos’ is not unambiguous. Literally translated it means ‘nephew’, but it may well have had other meanings and resonances, such as ‘pupil’ or ‘intellectual son’.

On the face of it therefore, Gualterus Arsenius is claiming to have Gemma as his uncle. However, it is likely that Gualterus liked to have Gemma’s name on his instruments for other reasons, especially commercial ones: Gemma was known throughout Europe thanks to his publications on mathematical subjects, which had been translated into several languages. Gualterus only started using the ‘nepos’ formula in 1557, two years after Gemma’s death, having in 1554 made an astrolabe under Gemma’s instruction signed ‘Authore Gemma Frisius et exaratu9 a Gualtero Arsenio Louanij 1554’, thus stating Gemma’s involvement but without any mention that he was his uncle.

The Stuttgart instrument is unique in being signed by the ‘nepotes’ or ‘nephews’ of Gemma Frisius, referring to a collaboration between Gualterus Arsenius and his pupil (and son?), Ferdinand Arsenius. Ferdinand started to make instruments in 1573 very much in the vein of Gualterus. His style of engraving is very similar to Gualterus’s, but differs from it in, for example, the italic letters, which look less elongated. The swashes of the letters on Ferdinand’s instruments also are much longer than those of Gualterus, sometimes even underlying parts of the next word – something rarely found in Gualterus’s hand. Gualterus died in 1580 and it is likely that most parts of the Stuttgart instrument were engraved by Ferdinand.

The Oxford instrument is clearly in the hand of Gualterus Arsenius and can be dated to around 1570. The main evidence for this attribution is the style of the engraving, which is mostly in the italic script. A close visual comparison of the engraving on the unsigned instrument with that on signed instruments by Arsenius, such as an astrolabe of 1566 in the Museo Arqueologíco in Madrid, reveals that in all respects the hand is identical.

Such a ‘palaeographical’ method of attribution is given further credibility when other engravers are included in the comparison. In the case of Gualterus Arsenius the only scope for confusion is with the hand of Adrianus Descrolieres, who matriculated at the University of Louvain in 1564 and then worked in the workshop of Arsenius until around 1571 when he started to make instruments on his own.

Descrolieres copied the instruments of his former master meticulously but always failed to equal them. His style of engraving looks imperfect or immature and lacks the spontaneity of Arsenius. Such observations and conclusions can only be made when looking at the instruments at very close quarters and preferably with a magnifying glass, which explains why several unsigned Descrolieres instruments have previously been misattributed to Arsenius.

Both the Oxford and Stuttgart instruments are probably copied from an unknown standard text on surveying which illustrates such a theodolite. This hypothesis is given extra strength by the fact that a third comparable theodolite exists, in the Museum of the History of Science in Florence. It was made by Petrus ab Aggere and is signed ‘Absolvit Toleti Petrus ab Aggere Mathematicus Regius Dni 1560’.

Very little is known about Petrus ab Aggere. It is possible that he is the same Aggere as appears in the roll of the University of Louvain for 1522: ‘Petrus filius Henrici de Aggere de Weestmalia’. Abraham Ortelius, the great cosmographer and map maker from Antwerp, credits Aggere, in an entry of his ‘Catalogus’, with a ‘world map, in the shape of an eagle, published in Mechelen’. If Aggere was a map maker, then this might explain his interest in constructing a theodolite.

Aggere’s theodolite is strikingly similar to both Arsenius instruments. It has a square base-plate with the letters ‘A’, ‘B’, ‘C’ and ‘D’ in the corners and three circular scales which enclose a wind rose with eight winds given in their usual Italian names. On top of the base-plate is a revolving circular plate with sighting vanes and pointer which moves over the scales on the base. The circular plate is also partly fretted so that the wind directions can be read below. An important difference, however, is the absence of a sundial. Instead, the surface of the circular plate is engraved with radiating lines running from the centre to edge, where a scale of degrees from 0 to 360 is engraved.

Gemma Frisius advocated the use of a horizontal graduated circle to survey land, since it allows the angle to be read directly at the edge of the plate. Ab Aggere’s square simple theodolite seems to be the oldest surviving example where Gemma’s method is incorporated. Although it is square, all its scales are circular, including even its shadow square.

The newly identified theodolite- sundial by Gualterus Arsenius is a very good example of the skill of the leading Louvain instrument maker in combining elegance with accuracy. Elegance was achieved by the refined style of engraving, both for the scales and their labels. The accuracy is suggested by the degree scale on the outer edge of the instrument, which is carefully subdivided to a quarter of a degree, an unusually fine division for an instrument from this period.

Koenraad Van Cleempoel