“Of course to make the bomb was simple enough for Weintraub.  He had an infernally complete laboratory in the cellar of his house, where he had made hundreds.  The problem was, how to make a bomb that would not look suspicious, and how to get it into the President’s private cabin.  He hit on the idea of binding it into the cover of a book.  How he came to choose that particular volume, I don’t know.”

“I think probably I gave him the idea quite innocently,” said Roger.  “He used to come in here a good deal and one day he asked me whether Mr. Wilson was a great reader.  I said that I believed he was, and then mentioned the Cromwell, which I had heard was one of Wilson’s favourite books.  Weintraub was much interested and said he must read the book some day.  I remember now that he stood in that alcove for some time, looking over it.”

“Well,” said Aubrey, “it must have seemed to him that luck was playing into his hands.  This man Metzger, who had been an assistant chef at the Octagon for years, was slated to go on board the George Washington with the party of cooks from that hotel who were to prepare the President’s meals.  Weintraub was informed of all this from someone higher up in the German spy organization.  Metzger, who was known as Messier at the hotel, was a very clever chef, and had fake passports as a Swiss citizen.  He was another tool of the organization.  By the original scheme there would have been no direct communication between Weintraub and Metzger, but the go-between was spotted by the Department of Justice on another count, and is now behind bars at Atlanta.

“It seems that Weintraub had conceived the idea that the least suspicious way of passing his messages to Metzger would be to slip them into a copy of some book—­a book little likely to be purchased—­ in a second-hand bookshop.  Metzger had been informed what the book was, but—­perhaps owing to the unexpected removal of the go-between—­ did not know in which shop he was to find it.  That explains why so many booksellers had inquiries from him recently for a copy of the Cromwell volume.

“Weintraub, of course, was not at all anxious to have any direct dealings with Metzger, as the druggist had a high regard for his own skin.  When the chef was finally informed where the bookshop was in which he was to see the book, he hurried over here.  Weintraub had picked out this shop not only because it was as unlikely as any place on earth to be suspected as a channel of spy codes, but also because he had your confidence and could drop in frequently without arousing surprise.  The first time Metzger came here happened to be the night I dined with you, as you remember.”

Roger nodded.  “He asked for the book, and to my surprise, it wasn’t there.”

“No:  for the excellent reason that Weintraub had taken it some days before, to measure it so he could build his infernal machine to fit, and also to have it rebound.  He needed the original binding as a case for his bomb.  The following night, as you told me, it came back.  He brought it himself, having provided himself with a key to your front door.”