Sorry, I should have been more specific.
The picture shows one like mine. In essence, if you know the trajectory of a projectile and charge for a given gun, you can elevate the barrel to have the projectile hit at a certain distance. The thing in the picture sat on a Hotchkiss machine gun, the end of the bubble level was raised so that if it were made level it would be that many degrees off the horizontal.
I picked up a couple of these ... the last one is an illustration from the 1894 Hotchkiss Catalog. I have one just like it.
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