Centering the Bubble

I stepped out to the low brick retaining wall at the edge of the garden, where an aluminum torpedo level lay flat across the mortar. Inside its horizontal vial, an air bubble floated three millimeters to the right of the dual index lines. I knelt beside it and tested the surface angle. The masonry sloped slightly downward. Searching along the joint, I gathered a thin flake of dark slate and wedged it beneath the tool's lower corner. The elevation shifted. The trapped bubble drifted steadily through the pale fluid until it locked squarely between the twin black markings. Equilibrium achieved.