What the heck, it's Saturday, I'm going to be busy, so are you, if the server logs are any indicators over time.

This is a Soviet-era aiming circle. They loved to make complicated aiming circles. They made pretty accurate instruments, too - with one artifact that made them less precise than western circles. The Russians based their circular measurement on a 6000 degree circle. Western armies initially used 360 degrees, like a standard compass (which is less accurate than the Russian measurement) but later shifted over to mils, of which there are 6400 in a circle. That being determined by the fact that a circle of 1 kilometer diameter has a circular measurement of 6400 meters - meaning that one mil of movement left or right describes a movement of 1 meter at 1 kilometer. This gives a nice easy way to determine all sorts of things when surveying, or aiming cannon. The Russian approach is slightly less accurate - but they tended to make up for that will volume of fire.

The aiming circle is essentially a simple theodolite - an instrument designed to measure angles, whether vertical or horizontal.

Orient the aiming circle on a known direction, whether using surveyed lines or a built-in compass to orient to north (applying that declination constant, of course - a measurement that accounts for the deviation of magnetic north from true north, as well as local and instrument variations, only good for the local region, but that's a long boring post I won't bother you with...).

Once you've done that, you can then apply the principle of "opposite interior angles are equal" and lay your guns so that all are pointed the same direction. Another somewhat boring post I probably won't write, but ya never know, I might get grumpy and decide to punish you with the gory details.

Anyway - this Russian aiming circle has an attachment on it, not normally used. Anyone want to hazard a guess as to its purpose? I will give a clue that should help out 'Murican artillerymen - the US approach to this problem was both simpler, yet periodically required depot-level services.

Here are two more pics, to help you in your pondering. A largish one.. And this one:

The item whose purpose you're trying to identify is the gizmo on the left side of the intstrument. What looks like a black knob is in fact an eyepiece. What looks like a silver knob on the right, is *also* an eyepiece - and the main one, that you look through when sighting the instrument. The gizmo we're concerned with is in fact hooked on to circle in such a way it blocks the main viewer completely - since for the purpose of the gizmo, it's simply a convenient place to mount it so that you can level the gizmo. The silver knob in the middle - *that's* a knob. They key clue here is... the plane of view of the gizmo is perpendicular to the plane of view of the basic instrument.