This gizmo is a dial sight. In US parlance, it's called a panoramic telescope. This particular one is the WWI-era sight for the Brit 18pdr howitzer.
To hit a target the gun can't see, without a lot of wasting rounds registering and adjusting, you need to have a few simple things.
Accurate target location. Accurate observer location. Accurate gun location, and a common grid to measure angles from. There's some other stuff, like accurate weapon, ammunition, and weather data, but that's the subject for a different, glaze-your-eyes post that will make people run away screaming in fear. Or get me sued as people break their noses as their heads hit the table as they fall asleep reading.
Anyway - you align the gun tube on a known azimuth. To do that, you use an aiming circle (director in commonwealth-speak) to align the gun and sight on a known azimuth. You set that azimuth on the sight, with the sight pointed to an aiming reference point, whether it's a collimator as used now (an instrument that simulates an infinity reference point but that can be placed close to the gun), aiming posts, (which, when aligned in the sight mean that you are looking at them straight on) or a distant aiming point, at least 1000m away (least desirable, bad weather is your enemy there).
Still interested? The rest is in the extended post.
Once you've done that - you can then point the tube where you want it. You calculate the the amount you need to move the tube left or right of the azimuth the gun is laid on. You send that data to the guns. The gunner sets that amount of deflection on the sight. He then looks through the sight, and moving the gun, not the sight, he brings the sight picture back to the aiming point. Voilą! You have now moved the gun tube to the azimuth of fire.
There are two approaches to this. One is lay by azimuth, where everything is done by compass azimuth, the other is lay by deflection, where instructions to the guns are given as left or right deflections from a zero line. This is the method the US uses. It is nominally more secure, because you aren't giving away compass bearings, which aid the enemy listening to your transmissions in trying to locate your guns. In an era of digitally encrypted commo, it's not as important, but our system is built around it. Many nations just use in the clear compass readings - which in the age of digitally encrypted commo is fine.
There are three ways in use that I'm aware of to measure the angles. The metric 6400 mil circle, the Russian arshin-based 6000 mil circle, and standard compass bearings. Sounds complicated, but it isn't. A circle one kilometer in diameter has a circumference of 6,400 meters. The arshin, a pre-metric Russian equivalent to the yard or meter, yields a circle of 6000 arshins. And standard compass bearings yield 360 degrees. The finer you can make your circular gradations, the more accurate you are at greater ranges. If you think about each marking unit as a ray proceeding from the center of the circle out to infinity, there is one meter between the rays at 1 kilometer. 10 meters at 10 kilometers, 20 meters at 20 kilometers. Those distances are even larger using the other systems. At long ranges, that begins to have an effect on accuracy.
Okay. I'm done now. If you're still here, congratulations! If not, well, I understand.
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