What sensation does Farquhar experience “with terrible suddenness” after he has been hanged?

He had fallen into the stream because the rope had broken,

"Then all at once, with terrible suddenness, the light about him shot upward with the noise of a loud splash; a frightful roaring was in his ears, and all was cold and dark. The power of thought was restored; he knew that the rope had broken and he had fallen into the stream."