before when people I wanted had flown from a house I have been able to
follow them by putting a guard over their mail-box. These letters,
which arrive regularly every week from Russia in the same handwriting,
they can come but from one person. At least, we shall now know the
name of the master of this house. Undoubtedly it is one of his letters
that the man placed here this morning. We may make a most important
discovery.'

"As he was talking he was picking at the lock with his knife, but he
was so impatient to reach the letters that he pressed too heavily on
the blade and it broke in his hand. I took a step backward and drove
my heel into the lock, and burst it open. The lid flew back, and we
pressed forward, and each ran his hand down into the letterbox. For a
moment we were both too startled to move. The box was empty.

"I do not know how long we stood staring stupidly at each other, but
it was Lyle who was the first to recover. He seized me by the arm and
pointed excitedly into the empty box.

"'Do you appreciate what that means?' he cried. 'It means that some
one has been here ahead of us. Some one has entered this house not
three hours before we came, since eleven o'clock this morning.'

"'It was the Russian servant!' I exclaimed.

"'The Russian servant has been under arrest at Scotland Yard,' Lyle
cried. 'He could not have taken the letters. Lord Arthur has been in
his cot at the hospital. That is his alibi. There is some one else,
some one we do not suspect, and that some one is the murderer. He came
back here either to obtain those letters because he knew they would