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It used to be really time consuming to make musket balls. You had to melt a lump of lead, pour it into a mold, wait for it to solidify, break the mold and check if it was spherical. Wash and repeat about thousands of times, depending on the size of your army.
William Watts, however, was inspired one day by seeing that raindrops became spherical as they fell from the sky. He wondered if lead would do the same thing, so he experimented and that's how he came up with the idea of a Shot Tower.
Shot Towers were tall hollow structures with a melting chamber on top. Lead was molten on top, then poured through a sieve. The lead droplets would fall about 260 meters onto a water-filled basin. This would solidify the lead and then they'd be tested for roundness.
Watts made a fortune from patenting and building these Shot Towers and were built around the world, making bullet-crafting a lot easier.