This instrument was designed for range-finding in gunnery, and was made by one of the people associated with the invention of this type of device, Jost B?rgi. In fact other mathematicians claimed to have designed instruments working on similar principles.
In use, one arm was adjusted to the length of a measured baseline, reduced to scale, and the other two aligned on the target from either end of this line. The range could then be taken from scales on the arms. The connection with gunnery is reinforced by scales engraved underneath which give the weight for a range of sizes of shot of different materials.
The instrument is simple but very finely made, and is engraved with the name of the original owner, Alberto Marchoni.