It's actually strangely heavy, I think the top part is solid aluminum besides the small hole for the center shaft. And it's pretty big, I should have put a ruler next to it. I'll update with size tomorrow. The short string is really weird for a plumb bob. I can't figure out why it would be so short.
Those are just a fad that is losing ground every day.
It's partly about the density, and also about the fact that the mercury sloshing around in it's chamber would dampen the swinging of the bob, causing it to settle more quickly. The mercury filled ones that I'm familiar with would be plated steel, with a largely cylindrical body. The brass turnips are usually not filled with mercury.
Brass--or anything containing copper, silver or gold--wouldn't be mercury-filled, as the mercury dissolves into the metal, forming a soft, brittle amalgam. If you're old enough, you may have some mercury/silver amalgam stuffed into your teeth.
I am old enough and I do have silver amalgam fillings.
Funny how, if a mercury-in-glass thermometer is dropped in a school, it takes shutting the school down and a hazmat team to make it safe, but it's perfectly safe in your mouth....