Many of the specialist instrumentation used by Royal Engineers was drawn from civilian surveyors’ equipment- there often being little difference in the tasks performed in both civilian and military spheres of life. Once such instrument was the gap measurer which was used, as its name would imply, for surveying the distance between two points such as the two sides of a gorge prior to building a bridge. An accurate calculation as to the distance was essential in ensuring the correct type of structure was chosen and the correct quantities of materials ordered with which to construct any bridge. Today we are looking at an example that most likely dates from the Second World War. It is a long black metal box with dials on the exterior: 

The device uses a pair of moving mirrors that are adjusted until the two points are both visible and the distance can then be read off a dial. One side of the instrument has two apertures for the mirrors, each with a sliding cover: 

Up close the mirrors can be seen inside the instrument: 

A metal fitting is attached to the opposite side: 

And a rotating dial which provides information in the distance being measured: 

The instrument is marked with its designation and a serial number: 

An instrument such as this was a precision object and so a robust carrying case was needed to ensure it did not get damaged. This took the form of a fibre box, covered in cloth and with heavy duty leather straps and buckles, together with a carry handle: 

Openings were left in the exterior of the box so the instrument could be used without removing it from the case: 

The interior has space for the gap measurer, together with a small compartment at one end: 

This contains spares to help maintain the device if it were to be damaged or come out of alignment: 

A small pair of wires is fitted under the lid in a small pair of carrying slots: 

These instruments were used for many years after the war and were also sold onto the civilian market where they would be just as useful as in a military context and it is most likely down this route that my example has come.