| Instrument | : | Preliminary Survey Transit 5129N (boussole theodolite) |
| Manufacturer | : | Keuffel & Esser |
| Country of origin | : | U.S.A. |
| Manufacturing year | : | 1919-1920 |
I've obtained this transit of American origin in 2008. Despite its age it is in remarkable good state. It still has most of its lacquered finish and a lovely patina at those places where it has gone. The instrument combines the functions of a theodolite with those of a compass. Basically it is an improvement on the earlier graphometer.
The name 'Transit' finds its origin in the fact that the instrument was capable of vertically rotating (transiting) the telescope to show the observer the opposite direction of his last measurement. This was a great advantage in, for instance, rail road construction back in the 19th and early 20th century. In general transits were less accurate than theodolites, but the mayor advantage of the transit was that, while traversing using the compass, only half of the stations were required (see fig. 21), although at the cost of some accuracy. Especially when setting out a route in unknown territory this method will speed up the whole process significantly. For this reason transits are still regularly used in pre-route surveys through dense forest or jungle where GPS fails due to poor reception. The method does however need a regular check of the magnetic deviation, which can be done by taking sun shots.
The instrument is listed in the 1921 K&E Catalogue as 'Preliminary Survey Transit No. 5129N.'.1 According to the price list of that year it would cost, including a box and tripod, US$205 (at that time approximately fl.695 or €315, roughly US$2200 in today's money or €3300 (November 2008)). This made it their cheapest transit with a compass and both horizontal and vertical limbs (model 5124 was without compass and vertical limb, model 5126 only had a vertical limb that would read down to 5 arc minutes). The diameters of the limbs are 5" (horizontal, 127mm) and 3½" (vertical, 89mm) and both can be read down to 1 arc minute. The compass has a 3½" needle and an adjustable limb, which allowed to correct for magnetic deviation (or otherwise to align the limb with the telescope).
The base is quite different from modern theodolites. It has four adjustment screws and instead of being on top of a flat tripod table, it is screwed directly on the threaded head of a tripod (in this case 3" x 12). The 1921 catalogue tells us that '...four-screw instruments are the favorites of Engineers...', but that the tribrach types '...are beginning to find increasing favor in engineering circles, as [it] possesses many distinct advantages...', resulting in '...a higher degree of accuracy...'. Their high precision levels and theodolites were for this reason '...always furnished with three levelling screws...'. The base has a large hole by which the instrument can be centred above a geodetic marker. The instrument is finally fastened by turning the four adjustment screws until they get locked, in the meanwhile keeping the instrument's primary axis vertical.
The '...8in, achromatic terrestrial...' telescope has an object glass of 1", a '...15 diameters...' magnifying power and fixed stadia hairs with a 1:100 ratio. The stadia hairs and reticule are made of cobweb. According to a Dutch book on surveying, the web of a Cross Orbweaver (Araneus Diadematus) was used for this purpose,2 although different species might have been used for this in the US. Of the four original hairs (one vertical and three horizontal), the vertical was missing when I obtained the instrument (see fig. 15). I decided to restore the reticule for which the telescope had to be taken apart. First the eyepiece was removed (this can simply be screwed off), then the centring ring of it by loosening the four plain screws around it. The reticule is fastened with four screws with holes in their sides that allow to turn them using a pin. The reticule has that much play that it will simply roll out of the telescope once the screws are removed and thus care should be taken not to damage the remaining hairs. For the latter I used a socket wrench to support the reticule in the process (see fig. 17). Up to here the process only took five minutes.
The reticule was placed on a flat surface and cleaned from old lacquer where the vertical hair was once fixated. Then a fresh 30 centimetres long non-adhesive silk thread of a cobweb (one of the threads the web was suspended with) was harvested in my garden, laid over the reticule while kept under tension and temporarily kept in place and under tension using two weights. Finally the thread was fixated with two small drops of colourless nail polish (see fig. 16, nail polish has the advantage that it dries within several minutes, while a small soft brush comes with it). After re-assembling the telescope the instrument had to be aligned again. For this a target at about 150 metres distance was sighted in two faces, after which the instrument was set at the average angles. Using the adjustment screws the reticule was then moved back to the target. After that the instrument works fine, but the hairs are visually off-centred. In order to get them centred again, the eyepiece centring ring needs to be adjusted, which will only affect the field of view, not the calibration (see fig. 19). Limited by the resolution of the circles, but aided by a magnifying glass, I managed to calibrate the horizontal and vertical collimation to about 15-30 arc-seconds (7-14 centimetres at 100 metres distance). In the meanwhile it was three hours later (the first attempt failed, the vertical hair broke while assembling, causing half an hour extra work) and the instrument is in working order again.
Attached to the telescope is a large plate level (60 arc seconds per graduation) which allows it to be used as a levelling instrument.
Although a case with plumb bob and oil bottle were supplied with the instrument (see fig. 23), it is far from complete. The case lacks its floor board on which the instrument would be mounted before storing it inside. The floor board would have measured approximately 7" x 8½" x 7/8", with a 3" x 12 thread. In addition to that it has no lens cap (1 1/8"), sun shade and adjustment tools. If you have any or all of these parts in spare, please let me know.
[1]: Catalogue of Keuffel & Esser Co., Manufacturers and Importers, Drawing Materials, Surveying Instruments, Measuring Tapes, 36th Edition, (New York, 1921), p. 321.
[2]: J.A. Muller, A. Scheffer, Landmeten en Waterpassen, (Haarlem, 1948), p. 44.