"Besides, they break the force of the waves. If it was not for them it would be impossible for any boat to come up close to the face of the house, and a heavy storm might even break down the wall altogether. A tremendous sea would roll in here in a westerly gale; and if it hadn't been for these rocks it would have been necessary to build the lower part of the house absolutely solid to resist the sea. It is possible that the rocks were higher than they now are when the place was first constructed, in which case the house might have been almost entirely hidden from sight. Well, we may as well go back again, Bertie; we know all there is to be known about this side."

They swam back into the tunnel, dressed, and went out.

"We have come out, Maria," Bertie called. "The coast is clear for you. The water is not so deep as we thought it was, and you can walk out to the point where the roof comes down on to the water without getting out of your depth."

It did not take them long to cut a number of switches to serve as brooms, and a couple of handles. They carried them up into the house, and lashed the switches firmly on to the handles. The work was rough, but the brooms when completed were large, and, although not strong enough for heavy work, would do well to sweep aside the thin layer of almost impalpable dust on the floor below.