on, likely to be exposed to wet. But this inference instantly
suggested another, which I was yesterday able to put to the test--thus."

He filled a tumbler with water, and, rolling up the document, dropped it
in. Immediately there began to appear on it a new set of characters of a
curious grey colour. In a few seconds Thorndyke lifted out the wet
paper, and held it up to the light, and now there was plainly visible an
inscription in transparent lettering, like a very distinct water-mark.
It was in printed Roman capitals, written across the other writing, and
read:

"THE PICKERDILLEY STUF IS UP THE CHIMBLY 416 WARDOUR ST 2ND FLOUR BACK
IT WAS HID BECOS OF OLD MOAKEYS JOOD MOAKEY IS A BLITER."

The Professor regarded the inscription with profound disfavour.

"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.

"Yes," he snorted; "but I am a scholar, sir, not a policeman