which surrounded the
house on the three sides. He put on his lamp again, but the light was not
powerful enough to carry the distance required, and he went forward at a
jog trot in the direction he had seen the figure disappear. He reached
the pines and went softly. Every now and again he stopped, and once he
could have sworn he heard the cracking of a twig ahead of him.

He started off at a run in pursuit, and now there was no mistaking the
fact that somebody was still in the wood. He heard the quick steps of his
quarry and then there was silence. He ran on, but must have overshot the
mark, for presently he heard a stealthy noise behind him. In a flash he
turned back.

"Who are you?" he said. "Stand out or I'll fire!"

There was no answer and he waited. He heard the scraping of a boot
against the brick-work and he knew that the intruder was climbing the
wall. He turned in the direction of the sound, but again found nothing.

Then from somewhere above him came such a trill of demoniacal laughter as
chilled his blood. The top of the wall was concealed by the overhanging
branch of a tree and his light was valueless.

"Come down," he shouted, "I've got you covered!"

Again came that terrible laugh, half-fear, half-derision, and a voice
shrill and harsh came down to him.

"Murderer! Murderer! You killed Thornton Lyne, damn you! I've kept this
for you--take it!"

Something came crashing through the trees, something small and round, a
splashing drop, as of water, fell on the back of Tarling's hand and he
shook it off with a cry, for it burnt like fire. He heard the mysterious
stranger drop from the coping of the wall and the sound of his swift
feet. He stooped and picked up the article which had been thrown at him.
It was a small bottle bearing a stained chemist's label and the word
"Vitriol."

CHAPTER XXVIII

THE THUMB-PRINT

It was ten o'clock in the morning, and Whiteside and Tarling were sitting
on a sofa in their shirt-sleeves, sipping their coffee. Tarling was
haggard and weary, in contrast to the dapper inspector of police. Though
the latter had been aroused from his bed in the early hours of the
morning, he at least had enjoyed a good night's sleep.

They sat in the room in which Mrs. Rider had been murdered, and the rusty
brown stains on the floor where Tarling had found her were eloquent of
the tragedy.

They sat sipping their coffee, neither man talking, and they maintained
this silence for