He accepted a cigarette, and lighted it carefully.

“Last night,” he resumed, “a member of P Division was on point duty outside the late Professor’s house, and two C.I.D. men were actually in the room where the safe is.  Result—­someone has put in at least an hour’s work on the lock, but it proved too tough a job!”

I stared at him amazedly.

“Someone has been at the lock!” I cried.  “But that is impossible, with two men in the room—­unless—­”

“They were both knocked on the head!”

“Both!  But by whom!  My God!  They are not—­”

“Oh, no!  It was done artistically.  They both came round about four o’clock this morning.”

“And who attacked them?”

“They had no idea.  Neither of them saw a thing!”

My amazement grew by leaps and bounds.  “But, Bristol, one of them must have seen the other succumb!”

“Both did!  Their statements tally exactly!”

“I quite fail to follow you.”

“That’s not surprising.  Listen:  When I got on the scene about five o’clock, Marden and West, the two C.I.D. men, had quite recovered their senses, though they were badly shaken, and one had a cracked skull.  The constable was conscious again, too.”

“What!  Was he attacked?”

“In exactly the same way!  I’ll give you Marden’s story, as he gave it to me a few minutes after the surgeon had done with him.  He said that they were sitting in the study, smoking, and with both windows wide open.  It was a fearfully hot night.”

“Did they have lights?”

“No.  West sat in an armchair near the writing-table; Marden sat by the window next to the door.  I had arranged that every hour one of them should go out to the gate and take the constable’s report.  It was just after Marden had been out at one o’clock that it happened.

“They were sitting as I tell you when Marden thought he heard a curious sort of noise from the gate.  West appeared to have heard nothing; but I have no doubt that it was the sound of the constable’s fall.  West’s pipe had gone out, and he struck a match to relight it.  As he did so, Marden saw him drop the match, clench both fists, and with eyes glaring in the moonlight and his teeth coming together with a snap, drop from his chair.

“Marden says that he was half up from his seat when something struck him on the back of the head with fearful force.  He remembered nothing more until he awoke, with the dawn creeping into the room, and heard West groaning somewhere beside him.  They both had badly damaged skulls with great bruises behind the ear.  It is instructive to note that their wounds corresponded almost to a fraction of an inch.  They had been stunned by someone who thoroughly understood his business, and with some heavy, blunt weapon.  A few minutes later came the man to relieve the constable; and the constable was found to have been treated in exactly the same way!”