"But how will you get the stone posts from the quarry to the derrick," continued Will. "We have no vessel in which they can be carried."
"With the timbers to be used in raising the wharf to the required height, we can make a raft on which two or three of the posts can be floated at a time. It can be towed by the Sylph," answered Bolly, who had evidently been over all these details.
"But you can't set these posts under water. If they don't happen to stand plumb, I don't see how you can help yourself," suggested Luke Bennington.
"But we can help ourselves, even if we have to ask the principal to send to Boston or New York for a submarine diver who knows how to lay stone," said Bolly. "I should be willing to take the job of setting the posts plumb without any outside assistance except the stone-cutters at the quarry. The bottom is nearly level, and we can obtain the exact pitch by measuring. We can cut a flat rock to fit the bottom."
"There is still another way," interposed Mr. Bridges.
"I have thought of a coffer-dam," added Bolly, glancing at the visitor.
"Exactly so; that is what I was thinking about," laughed the civil engineer.
"What's a coffer-dam?" demanded Dave Winslow.
"There are several ways to make one," answered Bolly. "We could make an island by filling in earth: then dig out a hole in it the same as we should for a foundation on shore. Another way is to sink a wooden or iron caisson, or box, to the bottom, where it would have to be puddled with clay. Then it would be necessary to pump out the water. When this is done we should have a chance to set the posts just as we should do it on shore."