This instrument is the predecessor of the pantometer and the even more modern optical square, and was used to set out right and 45 degree angles in the field.
Together with the surveyor's cross the equerre is one of the oldest angle measuring devices used in land surveying. Both were developed out of the Roman Groma.
Equerres have been made in varying shapes. Most commonly know are the octogonal ones, but cylindrical (not to be confused with the pantometer), cylindrical ones with a divided limb, square and spherical shapes are known to have existed as well.
Early and home-made equerres were made of wood, sometimes with brass visors mounted in them. Similar to the pantometer, equerres were at times equipped with a magnetic compass on top of the instrument to facilitate orientation in respect to magnetic north (see figure 4).
Both equerre's came in their original boxes. The earliest of the two came together with my Secrétan Egault type level. In order to fit its box, the handle of the 19th century equerre can easily be unscrewed and inserted through the hole in the upper end of the instrument. The 20th century equerre fits the box with the handle in place.
Usually equerres are divided in 8 directions with corresponding sets of visors. Two sets of visors are usually found; those with and those without a vertical hair. The ones without hair are narrow slits, just under 1 millimetre wide.
The others are narrow for one half, while the other half is much wider, typically 4-5 millimetres wide, and contains the hair. According to Carl Maximilian Bauernfeind (and other period authors) horse-hair was used for the visors (Bauernfeind 1862, 130). Opposite sides of the instrument always have the same type of visors, but in case of the ones with hair the opposite visors are inverted. The narrow slit opposite the wider one with the hair helps to get the hair into focus even without accumulating the eye (see figure 3 where the camera was focussed to infinity).
If a compass is present it usually is divided in whole degrees.
The compass on my 20th century equerry shows the directions of the winds as 'N', 'O', 'Z' and 'W' (Noord, Oost, Zuid and West, in English: North, East, South and West), indicating that the instrument was intended for the Dutch market.
The handle has a conic hole, to make it possible to attach the instrument to a tripod (similar to the one of the Graphometer) or staff.
Scales and accuracy
Apart from the compass there are no real scales on the equerre. Together the visors create sight lines of 0°, 45°, 90°, 135° and their complements.
Sighting a direction with a dioptre system like the equerre can typically be done with an accuracy of about 10 arcseconds (Bauernfeind 1862, 27-28), although this value seems to be a bit optimistic. The accuracy of a staked-out angle further depends on the quality of the craftsmanship with which the instrument was made, the experience of the surveyor, and the staff or tripod the instrument is used on.
Orienting the instrument in respect to (local) magnetic north could probably be done with an accuracy of half a degree.
It may be clear that great accuracy could not be achieved with these instruments.
The advantage over the surveyor's cross and Graphometer in my collection is that all directions are fixed while the surveyors cross can only be fixated at 90° (and 0° of course). The horse hair type of visor can be found on most 19th century simple surveying instruments as the 20th century Pantometer and Graphometer, while earlier instruments - as the before mentioned surveyor's cross - lacked this hair.
If you have any questions and/or remarks please let me know.
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Equerre 19th century Pantometer? 20th century Pantometer Pantometre à Lunette 17th c. Surveyor's Cross Pseudo Holland Circle Graphometer