What is shown here is called the barrel or cannon sundial noon. The gun under little is mounted on a marble base, 9 inches and has a lens that is placed above it. When it is noon, the lens can focus sunlight to melt the barrel of light to be put on fire.
Imagine if the gun was loaded, could be used to “fire” that fell asleep on your work desk before lunch. What a great alarm clock that dangerous, but more effective than wake you up!