Once in a while a man belonging to the beach patrol passed the chief engineer and the superintendent, reporting only that no signs of Harry had been found.
An hour thus passed.

Then, from over the water, as the lantern-bearing searchers were returning, a dull explosion boomed across the water.
"Great Scott!" quivered Tom.

"There they go at it again, Mr.Renshaw! Another section of the retaining wall has gone---blown up!".