Presently he found a spot that sounded more hollow than other sections.
“By Jove, I think I’m right,” he muttered. “But there seems to be no crack or crevice. The panel, if there is one, is most cleverly concealed.”
Persistently searching the wall, however, Nick finally discovered the head of a nail some six feet above the floor. It did not appear to be as dusty as the rest of the wall. He reached up and pressed it with his thumb.
This instantly brought a faint click from behind the sheathing.
A section of it about two feet square, so neatly fitted that the cracks were invisible, separated from the rest and swung outward under the impulse of a hidden spring.
It brought to light the foundation wall of the building, also a circular metal plate about fourteen inches in diameter, with a handle by which it could be swung downward parallel with the face of the wall.
Nick forced it down and discovered the opening of a tube through the wall, and in the tube a cylinder such as Patsy had seen in the subterranean chamber.
Nick instantly hit upon the truth, of course, and the mystery as to how the merchandise had been taken from the store ended then and there.
“A pneumatic tube,” he said to himself, noting the tight-fitting flange of felt around the end of the cylinder. “Similar to those of a cash system. The tube evidently runs[{39}] underground to another building, where there must be an engine and air pump for removing the air from the tube. That done, and this plate lowered, the cylinder would fly through the tube in an instant.”
Nick carefully noted the probable direction of the tube, then turned a knob in the metal end of the cylinder, from which he took, as he expected—the bundle seen under Lombard’s arm only ten minutes before.