He returned to the cave, took from his pocket a ball of string and a tape-measure, tied the string to the flint corner, fastened a pebble at the nineteenth metre and flung it toward the land side. The pebble at most reached the end of the path. “Idiot that I am!” thought Beautrelet. “Who reckoned by metres in those days? The figure 19 means 19 fathoms[9] or nothing!” [9] The _toise_, or fathom, measured 1.949 metres.—_Translator’s Note_. Having made the calculation, he ran out the twine, made a knot and felt about on the piece of wall for the exact and necessarily one point at which the knot, formed at 37 metres from the window of the Demoiselles, should touch the Fréfossé wall. In a few moments, the point of contact was established. With his free hand, he moved aside the leaves of mullein that had grown in the interstices. A cry escaped him. The knot, which he held pressed down with his fore-finger, was in the centre of a little cross carved in relief on a brick. And the sign that followed on the figure 19 in the document was a cross! It needed all his will-power to control the excitement with which he was overcome. Hurriedly, with convulsive fingers, he clutched the cross and, pressing upon it, turned it as he would have turned the spokes of a wheel. The brick heaved. He redoubled his effort; it moved no further. Then, without turning, he pressed harder. He at once felt the brick give way. And, suddenly, there was the click of a bolt that is released, the sound of a lock opening and, on the right of the brick, to the width of about a yard, the wall swung round on a pivot and revealed the orifice of an underground passage. Like a madman, Beautrelet seized the iron door in which the bricks were sealed, pulled it back, violently and closed it. Astonishment, delight, the fear of being surprised convulsed his face so as to render it unrecognizable. He beheld the awful vision of all that had happened there, in front of that door, during twenty centuries; of all those people, initiated into the great secret, who had penetrated through that issue: Celts, Gauls, Romans, Normans, Englishmen, Frenchmen, barons, dukes, kings—and, after all of them, Arsène Lupin—and, after Lupin, himself, Beautrelet. He felt that his brain was slipping away from him. His eyelids fluttered. He fell fainting and rolled to the bottom of the slope, to the very edge of the precipice. His task was done, at least the task which he was able to accomplish alone, with his unaided resources. That evening, he wrote a long letter to the chief of the detective service, giving a faithful account of the results of his investigations and revealing the secret of the Hollow Needle. He asked for assistance to complete his work and gave his address.