Aunt Jane's Nieces on the Ranch by L. Frank Baum

With the added light that now came from the gratings in the ceiling every object in the upper room was plainly visible. Runyon began his inspection in a methodical manner, starting at one corner and eyeing the inner wall on every inch of its surface. He tested each block at its corners and edges. The girls watched him listlessly, for they expected no result, having covered the same methods themselves.

At length Runyon was obliged to abandon the wall in despair.