What is this instrument for?

[Lang]

Terry is correct. I think I can make out the description on the side.

Could be used for all sorts of things but most useful to artillery shooting at distant high ground. They knew the horizontal distance from their position to the enemy feature by looking at the map. By using the Abney level they could get an angle and by a simple trig calculation find the exact height not just guessing off the contours. They then adjusted the gun range up to land at the right spot up the slope instead of at the bottom as would happen if they just shot the horizontal distance.

The Abney level was not as good as a theodolite but I should imagine the Forward Observation Officer would prefer to skulk in the bushes putting this up to his eye rather than stand up and erect a theodolite tripod!

They were very useful in the early days of railroad construction and the pioneering surveyors used them to mark out a realistic grade track through hilly country before going back to do it in detail with a theodolite.

Lang