HELP!

How do you calibrate a spirit level?

Went round the charity shops today, bought a Flanders and Swann CD, a cheap DVD or two, a magnifying glass like the one my Grandad used to look at stamps through, and this dinky object.

It is very sensitive, a bus ticket under one edge is enough to put the bubble outside the circle. The window sill was installed by Penicuik so I presume it is level, but how do I check?

And apart from that can anyone tell me where it would have originally been used. I mean it looks like it came out of the midget submarines.