folded up carefully the paper containing the hairs, and taking the
hat in both hands, as though it were some sacred vessel, ascended with
me to the laboratory on the next floor.

"Now, Polton," he said to his laboratory assistant, "we have here a
specimen for examination, and time is precious. First of all, we want
your patent dust-extractor."

The little man bustled to a cupboard and brought forth a singular
appliance, of his own manufacture, somewhat like a miniature vacuum
cleaner. It had been made from a bicycle foot-pump, by reversing the
piston-valve, and was fitted with a glass nozzle and a small detachable
glass receiver for collecting the dust, at the end of a flexible metal
tube.

"We will sample the dust from the outside first," said Thorndyke, laying
the hat upon the work-bench. "Are you ready, Polton?"

The assistant slipped his foot into the stirrup of the pump and worked
the handle vigorously, while Thorndyke drew the glass nozzle slowly
along the hat-brim under the curled edge. And as the nozzle passed
along, the white coating vanished as if by magic, leaving the felt
absolutely clean and black, and simultaneously the glass receiver became
clouded over with a white deposit.

"We will leave the other side for the police," said Thorndyke, and as
Polton ceased pumping he detached the receiver, and laid it on a sheet
of paper, on which he wrote in pencil, "Outside," and covered it with a
small bell-glass. A fresh receiver having been fitted on, the nozzle was
now drawn over the silk lining of the hat, and then through the space
behind the leather head-lining on one side; and now the dust that
collected in the receiver was much of the usual grey colour and fluffy
texture, and included two more hairs.

"And now," said Thorndyke, when the second receiver had been detached
and set aside, "we want a mould of the inside of the hat, and we must
make it by the quickest method; there is no time to make a paper mould.
It is a most astonishing head," he added, reaching down from a nail a
pair of large callipers, which he applied to the inside of the hat; "six
inches and nine-tenths long by six and six-tenths broad, which gives
us"--he made a rapid calculation on a scrap of paper--"the
extraordinarily high cephalic index of 95.6."

Polton now took possession of the hat, and, having stuck a band of wet
tissue-paper round the inside, mixed a small bowl of plaster-of-Paris,
and very dexterously ran a stream of the