Hydrometer, how are they made?

So I was knocking back a few beers with one of my non-homebrewing friends and he was asking about equipment, specifically the hydrometer and how it works. So I grabbed mine and explained how it measures specific gravity and why that's important in the brewing process.

He then asked: How are they made?

Like are they blown, cast, at what point is the lead dropped in and how do they get the scale in there.

I did a quick you tube search and came up with nothing, so does anyone have anything?

they add lead to balance it to be calibrated in water to be 1.000 and how do they actually form the glass? I would expect it to be blown into a mold.