Stone, steel, custom drip system.
I installed a pair of these water clocks (one dripping on sandstone, one on serpentine) at This Will Take Time in northern California. As they are both hooked into the program’s water system, the activity of the place will be marked through erosion on the faces of these boulders. The length of the piece is indeterminate, and the outcome is unknown. I’ll be checking on them yearly to track changes. The boulders weigh ~ 1 ton each.