I found these images after reading a page on the Anchor Steam Brewery website. A shot tower is for making spherical gunshot. The Selby Shot Tower shown above and below was located at Howard and 1st Street, circa 1870. More after the jump...
Owner Thomas H. Selby arrived in San Francisco from New York in 1849 and became its mayor twenty years later. By 1870, his 200-foot-tall shot tower at First and Howard was already as prominent a contributor to San Francisco's burgeoning industrial economy as it was to its skyline.
But how did this thing work, you ask?
In a shot tower, lead is heated until molten, then dropped through a copper sieve high up in the tower. The liquid lead solidifies as it falls and by surface tension forms tiny spherical balls. The partially cooled balls are caught at the floor of the tower in a water-filled basin. The now fully cooled balls are checked for roundness and sorted by size; those that are "out of round" are remelted. A slightly inclined table is used for checking roundness. To make larger shot sizes, a copper sieve with larger holes is used. However, the maximum size is limited by the height of the tower, because larger shot sizes must fall farther to cool. A polishing with a slight amount of graphite is necessary for lubrication and to prevent oxidation.