Again and much louder came the rat-tat-tat. We all gazed expectantly at the closed door. Glancing at Holmes, I saw his face turn rigid, and he leaned forward in intense excitement. Then suddenly came a low guggling, gargling sound, and a brisk drumming upon woodwork. Holmes sprang frantically across the room and pushed at the door. It was fastened on the inner side. Following his example, we threw ourselves upon it with all our weight. One hinge snapped, then the other, and down came the door with a crash. Rushing over it, we found ourselves in the inner room. It was empty.