“How long has it been going on?”
“Don’t know that, either. Mebby it’s been goin’ on for quite a spell, an’ that the roar of the stamps sort of smothered it. Now, when the stamps is all hung up, the thumpin’ can be heard plain.”
“It comes from the tanks,” put in Hawkins; “maybe it’s a leaky valve poundin’.”
“There’s nothing in the pipes,” said Burke. “The cyanide plant is out of commission until to-morrow. Sounds as though some one was striking a club against the side of a tank. We ought to be able to run it down and find out what causes it.”
Burke started off, mounting a plank incline that led to the lower row of tanks, and then moving slowly along a plank gangway that spanned the tanks’ tops. Merry, Clancy, and Ballard followed him.
It was difficult to decide just where the thumping sound came from. The hollow, resonant note was very deceiving. A little search, however, proved conclusive that it did not emanate from the lower tier of vats, so Burke and Merry and his chums mounted to the next tier.
While they were hunting over the gangway that crossed the second tier, they were suddenly startled by a yell from Hawkins. Looking upward, they saw the deputy sheriff standing on a ladder, visible from his waist up over the top of the big solution tank. His form was silhouetted against a background of starry sky, and he was making grotesque and excited gestures with his arms.
“I’ve found what you’re lookin’ for,” he called. “Bring a lantern and come here. It’s in the solution tank.”
“That’s as it should be,” laughed Burke; “the solution tank is the proper one to offer a solution of the mystery.”
A lantern was secured, and Burke and the three lads hastened to climb to the huge tank that topped all the others. Those below, including Mr. Bradlaugh, went around the edge of the massed tanks and gained the reservoir from the other side.