19th century J.-B. Tibaut Desimpelaere Lenoir typeThis level entered the collection in 2013. It was made in Belgium by J.-B. Tibaut Desimpelaere, most likely in the 19th century.
It's design is typical for what we would expect from the 19th and early 20th century, very similar to the Egault level and Société des Lunetiers theodolite in the collection: all lacquered brass.
The level is the so called niveau cercle composé (a compose circular level), a type of level that has a circular frame that supports the telescope while it revolves around its primary axis. There is thus no need for a high quality primary axis. Instead the flatness of the circular frame has to be of reasonable quality and the same accounts for the flatness and thickness of the two blocks that hold the telescope and support the vial on top of it. This type of level with its typical circular base was invented by Étienne Lenoir (1744–1832) in 1820 and further improved by Dutch geodetic instrument firm Bayens in 1825.1
The relatively large diameter of the circular frame would have made it easier to achieve the required accuracies than with the short primary axis of period instruments.
Similar to the Secrétan Egault level the instrument is levelled using the three tribrach adjusment screws at the base. There is no tilting screw (like the 1926 Wild NKII) or compensator (like the Wild NA2).
The vial is similar in size as with the Secrétan Egault level in the collection. It lies loose on top of the telescope and is held in place by a single central notch.
It is difficult to say what kind of accuracies can be achieved with an instrument like this. the vial is divided in three major intervals, each subdivided in five minor intervals (see figure 6). The major intervals represent 5 arc minutes of tilt, making the minor intervals single arc minutes or just over 7 millimetres at 25 metres distance.
In order to compensate for instrumental errors the Lenoir type of level allows the telescope to be rotated along its longitudinal axis. In this way any error in vertical collimation of the reticle will be compensated for.
In addition to that the level can be reversed on the telescope, by which any errors of the plate level will be compensated for as well. Downside of the system was that the telescope and level were not attached to each other or to the base and when reversing dirt and sand grains could easily undo the alignment between them.2
The main downside to the Lenoir design is that all parts are laid loose on top of each other. Being held in place by a central notch only means that the vial and telescope do not have a proper mutual alignment (i.e. pointing in the same direction), while it is very easy to knock off the vial or even the combination of vial and telescope.
The size of the cross-hairs do not add to the accuracy. Instead of an etched glass reticle or a diaphragm with spider threads this instrument is equipped with much thicker cross-hairs, possibly human or horse hair (see figure 3). The thickness of them would make it more difficult to achieve a proper reading. It could however be that the current cross-hairs are later replacements.
In 1929 a Dutch author on land surveying wrote that this type of levelling instrument found little use in the field.3 By the early 1940s it was regarded outdated, but still in use.4 Finally in 1956 another Dutch author wrote that the Lenoir type of level would not be discussed in his work as it was about time it disappeared from the profession.5
The J.-B. Tibaut Desimpelaere Lenoir type of level came complete with its original box, vial and sun shade. Apart from the broken cross-hairs it is still in working order. The base of the instrument, the instrument box and the vial box are all marked with what possibly is the serial number 488 (see figure 1, figure 4, figure 6 and figure 11). In addition to that the frame has the text "S.C.V. No.33" engraved on it (see figure 10). This seems to have been the inventory number of the original owner, but so far this remains unclear.
Notes: G.A. van Kerkwijk, Verhandeling over het Waterpassen en het gebruik van den Barometer, tot het Meten van Hoogten, ('s Gravenhage / Amsterdam 1828), pp. 109,114.
: J.A. Muller, A. Scheffer, Landmeten en Waterpassen, (Haarlem, 1948), p. 273.
: H.J. van Leusen, Landmeten en Waterpassen, (Delft, 1929), p.135.
: W. Schermerhorn, H.J. van Steenis, Leerboek der Landmeetkunde voor het middelbaar en hoger technisch onderwijs en voor de praktijk., (Amsterdam, 1941), p.114.
: W. Geerts, F.H. Knoups, Grondwerken, Terreinmeten en Waterpassen, (Haarlem, Antwerpen, Djakarta, 1956), p.3.
If you have any questions and/or remarks please let me know.
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