"It's well-founded; but really it's a very simple matter. The oldest papers presented at first, when they were new, a smooth, glossy surface. But soon wire-marks appeared in them, crossed at regular intervals by perpendicular lines, both reproducing the impression of the metal trellis on which the paste was spread. In the fourteenth century they had the idea of utilising this reproduction by making it a mark of the source or mill which the paper came from. With this object in view, they embroidered in brass wire on the trellis mould, initials, words, and all kinds of emblems: these are the water-marks. Every water-marked sheet of paper carries in itself its birth-certificate; but the difficulty is to decipher it. It requires a little practice: the pot, the eagle, the bell..."
Theophrastus opened his pocket-book and held out his scrap of paper with trembling fingers.
"Could you tell me the exact age of this document?" he said.
Ambrose put on his spectacles and held the paper up to the light.
"There's a date," he said. "172... The last figure is missing. It would be a paper of the eighteenth century then. Given the date within ten years, our task becomes very simple."
"Oh, I saw the date," said Theophrastus quickly. "But is this really an eighteenth-century paper? Isn't the date false? That's what I want to know."
Ambrose pointed to the middle of the scrap.
"Look," he said.
Theophrastus looked; but he saw nothing. Then Ambrose lighted a little lamp and threw its light on the document. In holding the scrap of paper between one's eyes and the lamp one distinguished in the middle of it a kind of crown.
"This paper's extremely rare, Theophrastus!" cried Ambrose in considerable excitement. "This water-mark is almost unknown, for very little of it was manufactured. The water-mark is called 'The Crown of Thorns.' This paper, my dear Theophrastus, is exactly of the year 1721."