19th century Secrétan à Paris Egault type level

The  Secrétan à Paris Egault type level.
Figure 1: The Secrétan à Paris Egault type level.
I've obtained this level during (or rather at the end of) the 2009 SIS conference in Paris, France. On my question where to buy old instruments one of the French members told me to go to the "Marché aux Puces" (flea market) at Porte de Vanves in the south of Paris. The market is only in the weekends and I was told to go early, 7am at the latest, as that is when they start displaying their merchandise. So despite the late hours of the evening before (we had dinner on board of a canal boat on the Seine) I got up at 6am to get ready to take the metro to Porte des Vanves.

Initially not many instruments showed up, apart from several dividers and reduction compasses and the odd equerre. After the first trip over the market I decided to go back to one of the reduction compasses. After buying it I turned around and looked straight into an open van where the owner had just moved a box to the exit.

Being quite old and simple I suspected it might contain an instrument. Even so I was rather surprised when I opened it. Not only was there an Egault type level in it, it also seemed very complete, well maintained, unpolished and with a beautiful patina. Not only the level was in it, but also its lens-cap, sun-shade and a second box containing an equerre (see figure 6). After a bit of negotiation we came to a very affordable price for the set and now it's here on display.

The  Secrétan à Paris Egault type level from the other side.
Figure 2: The Secrétan à Paris Egault type level from the other side.
In design the instrument is quite similar to the 1869 theodolite by Société de Lunetiers in my collection. The angular shapes in the base, the double rimmed adjustment screws and the etched reticle can all be found on both instruments.

The 35 centimetres telescope is equipped with an etched reticle, showing vertical stadia hairs (see figure 12). Although uncommon on modern instruments, these vertical stadia hairs were found on quite a few instruments of European origin in those days. They were used with a horizontally held stadia rod and eliminated the negative influence of refraction, resulting in more accurate readings.

Vertical stadia hairs were still in use in the third quarter of the 20th century, especially on plane table alidades, of which some models allowed to turn the stadia hairs by 90 degrees.1

Secrétan à Paris

The label in the box.
Figure 3: The label in the box.
The instrument is of French origin, signed Secrétan à Paris (see figure 4). Secrétan was professor of optics at Lausanne, worked for the Paris observatory and was probably joined by his son and nephew, Auguste and Georges Secrétan who later (as per 1855) worked under the name Maison Lerebours & Secrétan.2 It is the latter name that can be read on the label inside the box (see figure 3).

The label however is not the original one as handwriting disappears behind it and remains of an older label are still visible. It is for this reason that I estimate that the instrument was made by Marc-Francois-Louis Secrétan before 1855 (As this is when the Maison Lerebours & Secretan signature was introduced2). On the label even more history is visible; the successor of Maison Lerebours & Secrétan, C. Epry, stamped his name on it and struck through the 11 Chaussée-d'Antin address. Epry made telescopes and succeeded the Ancienne Maison Lerebours et Secretan, Fondée en 1789 in the early 20th century.3,4

The plate vial with the Secrétan à Paris signature.
Figure 4: The plate vial with the Secrétan à Paris signature.
Accuracy
Like the Tibaut Desimpelaere level and 1965 Wild NK01 the instrument has to be levelled using its tribrach levelling screws. There is no tilting screw (like the 1926 Wild NKII) or compensator (like the Wild NA2). It is difficult to say what kind of accuracies can be achieved with an instrument like this. The 7 centimetres plate level (see figure 4) is divided with a scale running from 0 to 3, each of which are divided again in 3 subdivisions. The main divisions resemble a tilt of approximately 2 arc minutes per division (1 centimetre at 17 metres). Each subdivision then corresponds with 40 arc seconds (5 millimetres per 25 metres, the latter being a normal levelling distance). The subdivisions are spaced at 3 millimetre intervals and I estimate that levelling can be done with an accuracy of about 1 millimetre per 25 metres, about as accurate as the 1965 Wild NK01.

In order to compensate for instrumental errors the Egualt type of level allows the telescope to be rotated along its longitudinal axis (see figure 10). In this way any error in vertical collimation of the reticle will be compensated for. In addition to that the telescope can be reversed on the base, 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 and when reversing dirt and sand grains could easily undo their mutual alignment.5

Although still explained in survey manuals by the middle of the 20th century this type of level was considered outdated by that time. They were only discussed because they still existed and sometimes were used in the field, but - as a Dutch author wrote - the fact they were described was in no way a plead for their continued existence.5

Notes

[1]: Staley, W.W. Introduction to mine surveying, (Stanford, 1964)), pp. 246-259.
[2]: Websters Signature Database on Secretan: http://historydb.adlerplanetarium.org/signatures/
[3]: Brenni, P. '19th Century French Scientifi'c Instrument Makers, III: Lerebours et Secretan', in: Bulletin of the Scientific Instrument Society, No. 40 (1994), pp. 3-6.
[4]: Websters Signature Database on Epry: http://historydb.adlerplanetarium.org/signatures/
[5]: Muller, J.A. en Scheffer, A., Landmeten en Waterpassen, (Haarlem, 1948), p. 273.

If you have any questions and/or remarks please let me know.

The instrument on its box.
Figure 5: The instrument on its box.
 
The instrument in the box, together with the equerre box.
Figure 6: The instrument in the box, together with the equerre box.

The adjustments of the Egault type level.
Figure 7: The adjustments of the Egault type level.
 
The horizontal drive screw and clamping mechanism.
Figure 8: The horizontal drive screw and clamping mechanism.

The telescope can easily be taken from the yokes.
Figure 9: The telescope can easily be taken from the yokes.
 
The telescope of the Egault type level can be used in four different orientations.
Figure 10: The telescope of the Egault type level can be used in four different orientations.

The sun shade mounted on the level.
Figure 11: The sun shade mounted on the level.
 
The inverted view through the telescope showing etched vertical stadia hairs.
Figure 12: The inverted view through the telescope showing etched vertical stadia hairs.

Surveyor's crosses Geodetic Sextants Theodolites Total Stations Levels Standards Tools Firms
20th c. hydrostatic level 19th c. water bottle level 19th c. Secrétan Egault 19th c. Tibaut Lenoir 1928 Carl Zeiss Nivellier I 1926 Wild NKII 1924 Carl Zeiss Nivellier II 1948 Wild N1 1932 Carl Zeiss Nivellier III 1951 Wild N2 1965 Wild NK01 1965 Wild NK10 1961 Wild N3 1970 Wild NK2 1977 Wild N3 1999 Wild NA2-GPM3 20th c. Cowley 1960s Jenoptik Koni 007 1960s Zeiss Opton Ni 2