the strange blue orb

This is a strange item. At first I thought it was a target ball, then a lightning rod ball, now I have no idea. The whole ball stands about 3 1/4" tall. On the top is a rusted lid, which is about an inch in diameter. On the bottom is an interesting little node. It is round and then flat on the bottom. It extrudes about a quarter of an inch and has some screw thread wrapped around. The bottom is closed, and the top lid has a small hole in it and as stated before, is in rusted condition. The color is a nice sort of teal blue and it catches the light fantastically. If anyone can help me identify this strange little piece, I would really appreciate it.