To discover something is nothing. To establish the truth of your discovery is everything. Now how was I to go to Hanover, obtain permission to visit the _Herrenhausen_ and be alone in the Knight's Hall for the necessary time. You can imagine I had no intention of putting some palace curator on the track I had just found. It was then that an idea occurred to me which I will disclose to you as a proof of the value of coffee in deductive reasoning. You will remember that when I was studying the question of the employment of French artists by German princes in the seventeenth and eighteenth centuries I discovered that the locksmith's work had been given by the Elector Ernest-Augustus of Hanover to a Catalan named Giroud who had also worked for the Grand Duke of Lautenburg. This Giroud had even had difficulties with Ernest-Augustus over his accounts. At that time I had only cast a cursory glance at the file dealing with the case. It was necessary for me to examine it more carefully. Perhaps I should be able to find something about the system of secret springs installed by Giroud at the _Herrenhausen_. I decided to clear the matter up then and there. It was just after midnight I put an electric torch in my pocket and quietly left my room. At that moment I thought I heard a faint noise in the deserted corridor. "Come," I thought, "I can't let myself be scared like this by old papers!" When I got to the library I was disagreeably surprised to find the lights on. Professor Cyrus Beck was hard at work covering a black board with his formulæ and only stopping to consult five or six treatises open in front of him. There was, of course, nothing unusual about my appearance there. I had often gone down late at night to the library to clear up some point in my next day's lesson. All the same he looked at me with that suspicious air of the savant who always thinks you're going to rob him of something. Two or three pleasant words quickly reassured him. He condescended to confide to me that he was at a decisive moment in his experiments and that the next day, without doubt, perhaps that very night.... Through the open door came the noise of his furnaces, roaring like chimneys on fire. I thought it unwise to tell him that I, too, had reached the same stage as himself in another affair. Besides, almost at once he put away his books, folded up his notes, rubbed out his formulæ, wished me good-night and went. I was eager for his departure as I had already found what I wanted. With a sureness of method which astonished me I had put my hand straight off on the vital document, a bill of Giroud's, dated 1682, and addressed to Ernest-Augustus. It was a long bill, but I found the following item at once: For the chimney-place of the Baron's Hall, six springs, in my name, at one hundred and fifty livres the spring.--Total 900 livres.