"How do you suppose this was done?" he asked gloomily.
"I will show you," said Thorndyke. "I have prepared a piece of paper to demonstrate the process to Dr. Jervis. It is exceedingly simple."
He fetched from the office a small plate of glass, and a photographic dish in which a piece of thin notepaper was soaking in water.
"This paper," said Thorndyke, lifting it out and laying it on the glass, "has been soaking all night, and is now quite pulpy."
He spread a dry sheet of paper over the wet one, and on the former wrote heavily with a hard pencil, "Moakey is a bliter." On lifting the upper sheet, the writing was seen to be transferred in a deep grey to the wet paper, and when the latter was held up to the light the inscription stood out clear and transparent as if written with oil.
"When this dries," said Thorndyke, "the writing will completely disappear, but it will reappear whenever the paper is again wetted."
The Professor nodded.
"Very ingenious," said he—"a sort of artificial palimpsest, in fact. But I do not understand how that illiterate man could have written in the difficult Moabite script."
"He did not," said Thorndyke. "The 'cryptogram' was probably written by one of the leaders of the gang, who, no doubt, supplied copies to the other members to use instead of blank paper for secret communications. The object of the Moabite writing was evidently to divert attention from the paper itself, in case the communication fell into the wrong hands, and I must say it seems to have answered its purpose very well."
The Professor started, stung by the sudden recollection of his labours.