nature as to be made only by the trigger of a pistol falling from the
nerveless hand of a suicide. He intended to throw the weapon far out of
the window, but had not the strength. It might have fallen into the
carriage. As a matter of fact, it bounced away from the line and lies
among the grass about ten feet six inches from the outside rail. The
only question that now remains is where the deed was committed, and the
exact present position of the pistol reckoned in miles from London, but
that, fortunately, is too simple to even need explanation."

"Great heavens, Sherlaw!" I cried. "How can you call that simple? It
seems to me impossible to compute."

We were now flying over Northern London, and the great detective leaned
back with every sign of _ennui_, closing his eyes. At last he
spoke wearily:

"It is really too elementary, Whatson, but I am always willing to
oblige a friend. I shall be relieved, however, when you are able to
work out the A B C of detection for yourself, although I shall never
object to helping you with the words of more than three syllables.
Having made up his mind to commit suicide, Kipson naturally intended to
do it before he reached Brewster, because tickets are again examined at
that point. When the train began to stop at the signal near Pegram, he
came to the false conclusion that it was stopping at Brewster. The fact
that the shot was not heard is accounted for by the screech of the air-
brake, added to the noise of the train. Probably the whistle was also
sounding at the same moment. The train being a fast express would stop
as near the signal as possible. The air-brake will stop a train in
twice its own length. Call it three times in this case. Very well. At
three times the length of this train from the signalpost towards