"'_Let us first consider what we do understand_: that is the right end of Reason. We will next consider what we do not understand. No one is found in the guard's-van. The shock undoubtedly hurled the guard into the sand. _All that is certain._ Now, after having stripped Signor Petito of his clothes, you sat down on the embankment and read his papers. When you raised your head, the railway-carriage was no longer there. Well, since there was a slope and since there was a wind, which waggled the head of M. Petito at the window, the carriage, after having glided down to H, found itself once more on the line A B a little higher up than H on the side of B, where the staff of the station have by now certainly found it. _Do you understand now? Do you understand everything, except that you haven't seen the train pass K? Since everything is thus explained, it must be that that was how things happened._ Now I only seek how you were not able to see the train pass K. That which it is impossible to explain in the case of five persons at A or at B may very well be explained in the case of one at K.' "'I am waiting,' said M. Longuet. "I chuckled--and truly there was reason to chuckle--and went on, 'There are moments when you are deaf, M. Longuet?' "'There certainly are,' said M. Longuet. "'Suppose you were deaf during the moment you were waiting for the train at K, _then you would not have heard it_.' "'No, but I should have seen it.' "We have already arrived at the fact that you did not _hear it_. That is a considerable advance! God bless you, M. Longuet! God bless you!' (M. Longuet was sneezing.) "M. Longuet thanked me for my pious wish, and since he continued to sneeze, I took out my watch from _his pocket_ (he had already stolen it from me), and I said to him: 'Do you know, M. Longuet, how long a single one of your sneezes lasts, that is to say, how long you remain with your head bent while you sneeze?... Three seconds!... That is to say a second and two-fifths longer than is required to fail to see an express with four carriages pass in front of you, which is going sixty miles an hour. M. Longuet, the express has disappeared, or rather seemed to disappear, _because you were deaf and had a cold_!' "M. Longuet threw up his arms wildly towards the ceiling of the Catacombs. CHAPTER XXV M. MIFROID TAKES THE LEAD "When M. Longuet had recovered from the emotion with which my explanation of the disappearance of the express had filled him, he embraced me and handed me a revolver which he had found in the pocket of Signor Petito. He did not wish to keep it on him. He was desirous that I should be able, at need, to defend myself against the eccentricities of which he feared, on grounds based, alas! on a too real experience, the dangerous return. For the same reason he entrusted to me a large knife which had also come from the pocket of Signor Petito.