ent, not only to diminish the noise, but also to
prevent any traces of the explosive from being left on the dagger. As to
the former, I think I can give you the murderer's name; but we had
better take the evidence in order. You may remember," he continued,
"that when Dr. Jervis stood as if winding the clock, I chalked a mark on
the floor where he stood. Now, standing on that marked spot, and looking
out of the open window, I could see two of the windows of a house nearly
opposite. They were the second- and third-floor windows of No. 6,
Cotman Street. The second-floor is occupied by a firm of architects; the
third-floor by a commission agent named Thomas Barlow. I called on Mr.
Barlow, but before describing my visit, I will refer to another matter.
You haven't those threatening letters about you, I suppose?"

"Yes, I have," said the inspector; and he drew forth a wallet from his
breast-pocket.

"Lot us take the first one, then," said Thorndyke. "You see that the
paper and envelope are of the very commonest, and the writing
illiterate. But the ink does not agree with this. Illiterate people
usually buy their ink in penny bottles. Now, this envelope is addressed
with Draper's dichroic ink--a superior office ink, sold only in large
bottles--and the red ink in which the note is written is an unfixed,
scarlet ink, such as is used by draughtsmen, and has been used, as you
can see, in a stylographic pen. But the most interesting thing about
this letter is the design drawn at the top. In an artistic sense, the
man could not draw, and the anatomical details of the skull are
ridiculous. Yet the drawing is very neat. It has the clean, wiry line of
a machine drawing, and is done with a steady, practised hand. It is also
perfectly symmetrical; the skull, for instance, is exactly in the
centre, and, when we examine it through a lens, we see why it is so, for
we discover traces of a pencilled centre-line and ruled cross-lines.
Moreover, the lens reveals a tiny particle of draughtsman's soft, red,
rubber, with which the pencil lines were taken out; and all these facts,
taken together, suggest that the drawing was made by someone accustomed
to making accurate mechanical drawings. And now we will return to Mr.
Barlow. He was out when I called, but I took the liberty of glancing
round the office, and this is what I saw. On the mantelshelf was a
twelve-inch flat boxwood rule, such as engineers use, a piece of soft,
red rubber, and a stone