Clinometer? Used to determine the degree of grade.
Don't ya need a built in "bubble" level to determine degree of grade?
The description at the worthpoint link suggested the same as I did .....
This item may have been for use in map-making for a cartographer or as a surveyor's instrument.
I'll hafta post a picture of what I think is a RR "track laying" clinometer, .... at least I know for a fact that it was made in Railroad, Pennsylvania.