Quite without meaning to.
Oswald went up on the roof, before breakfast, to see if he could find the hole where the rain had come in.

He did not find any hole, but he found the cricket ball jammed in the top of a gutter pipe which he afterwards knew ran down inside the wall of the house and ran into the moat below.

It seems a silly dodge, but so it was.
When the men went up after breakfast to see what had caused the flood they said there must have been a good half-foot of water on the leads the night before for it to have risen high enough to go above the edge of the lead, and of course when it got above the lead there was nothing to stop it running down under it, and soaking through the ceiling.

The parapet and the roofs kept it from tumbling off down the sides of the house in the natural way.

They said there must have been some obstruction in the pipe which ran down into the house, but whatever it was the water had washed it away, for they put wires down, and the pipe was quite clear.
While we were being told this Oswald's trembling fingers felt at the wet cricket ball in his pocket.