fortunately the train had stopped near a station. a short walk brought us to a garage where we were able to obtain a car, and half an hour later we were spinning rapidly back to london. then, and not till then, did poirot deign to satisfy my curiosity.

"you do not see? no more did i. but i see now. hastings, i was being got out of the way."

"what!"

"yes. very cleverly. both the place and the method were chosen with great knowledge and acumen. they were afraid of me."

"who were?"

"those four geniuses who have banded themselves together to work outside the law. a chinaman, an american, a frenchwoman, and—another. pray the good god we arrive back in time, hastings."

"you think there is danger to our visitor?"

"i am sure of it."

mrs. pearson greeted us on arrival. brushing aside her ecstasies of astonishment on beholding poirot, we asked for information. it was reassuring. no one had called, and our guest had not made any sign.

with a sigh of relief we went up to the rooms. poirot crossed the outer one and went through to the inner one. then he called me, his voice strangely agitated.

"hastings, he's dead."

i came running to join him. the man was lying as we had left him, but he was dead, and had been dead some time. i rushed out for a doctor. ridgeway, i knew, would not have returned yet. i found one almost immediately, and brought him back with me.

"he's dead right enough, poor chap. tramp you've been befriending, eh?"

"something of the kind," said poirot evasively. "what was the cause of death, doctor?"

"hard to say. might have been some kind of fit. there are signs of asphyxiation. no gas laid on, is there?"

"no, electric light—nothing else."

"and both windows wide open, too. been dead about two hours, i should say. you'll notify the proper people, won't you?"

he took his departure. poirot did some necessary telephoning. finally, somewhat to my surprise, he rang up our old friend inspector japp, and asked him if he could possibly come round.

no sooner were these proceedings completed than mrs. pearson appeared, her eyes as round as saucers.

"there's a man here from 'anwell—from the 'sylum. did you ever? shall i show him up?"

we signified assent, and a big burly man in uniform was ushered in.

"'morning, gentlemen," he said cheerfully. "i've got reason to believe you've got one of my birds here. escaped last night, he did."

"he was here," said poirot quietly.

"not got away again, has he?" asked the keeper, with some concern.

"he is dead."

the man looked more relieved than otherwise.

"you don't say so. well, i dare say it's best for all parties."

"was he—dangerous?"

"'omicidal, d'you mean? oh, no. 'armless enough. persecution mania very acute. full of secret societies from china that had got him shut up. they're all the same."

i shuddered.

"how long had he been shut up?" asked poirot.

"a matter of two years now."

"i see," said poirot quietly. "it never occurred to anybody that he might—be sane?"

the keeper permitted himself to laugh.

"if he was sane, what would he be doing in a lunatic asylum? they all say they're sane, you know."

poirot said no more. he took the man in to see the body. the identification came immediately.

"that's him—right enough," said the keeper callously; "funny sort of bloke, ain't he? well, gentlemen, i had best go off now and make arrangements under the circumstances. we won't trouble you with the corpse much longer. if there's a hinquest, you will have to appear at it, i dare say. good morning, sir."

with a rather uncouth bow he shambled out of the room.

a few minutes later japp arrived. the scotland yard inspector was jaunty and dapper as usual.

"here i am moosior poirot. what can i do for you? thought you were off to the coral strands of somewhere or other to-day?"

"my good japp, i want to know if you have ever seen this man before."

he led japp into the bedroom. the inspector stared down at the figure on the bed with a puzzled face.

"let me see now—he seems sort of familiar—and i pride myself on my memory, too. why, god bless my soul, it's mayerling!"

"and who is—or was—mayerling?"

"secret service chap—not one of our people. went to russia five years ago. never heard of again. always thought the bolshies had done him in."

"it all fits in," said poirot, when japp had taken his leave, "except for the fact that he seems to have died a natural death."

he stood looking down on the motionless figure with a dissatisfied frown. a puff of wind set the window-curtains flying out, and he looked up sharply.

"i suppose you opened the windows when you laid him down on the bed, hastings?"

"no, i didn't," i replied. "as far as i remember, they were shut."

poirot lifted his head suddenly.

"shut—and now they are open. what can that mean?"

"somebody came in that way," i suggested.

"possibly," agreed poirot, but he spoke absently and without conviction. after a minute or two he said:

"that is not exactly the point i had in mind, hastings. if only one window was open it would not intrigue me so much. it is both windows being open that strikes me as curious."

he hurried into the other room.

"the sitting-room window is open, too. that also we left shut. ah!"

he bent over the dead man, examining the corners of the mouth minutely. then he looked up suddenly.

"he has been gagged, hastings. gagged and then poisoned."

"good heavens!" i exclaimed, shocked. "i suppose we shall find out all about it from the post-mortem."

"we shall find out nothing. he was killed by inhaling strong prussic acid. it was jammed right under his nose. then the murderer went away again, first opening all the windows. hydrocyanic acid is exceedingly volatile, but it has a pronounced smell of bitter almonds. with no trace of the smell to guide them, and no suspicion of foul play, death would be put down to some natural cause by the doctors. so this man was in the secret service, hastings. and five years ago he disappeared in russia."

"the last two years he's been in the asylum," i said. "but what of the three years before that?"

poirot shook his head, and then caught my arm.

"the clock, hastings, look at the clock."

i followed his gaze to the mantelpiece. the clock had stopped at four o'clock.

"mon ami, some one has tampered with it. it had still three days to run. it is an eight-day clock, you comprehend?"

"but what should they want to do that for? some idea of a false scent by making the crime appear to have taken place at four o'clock?"

"no, no; rearrange your ideas, mon ami. exercise your little gray cells. you are mayerling. you hear something, perhaps—and you know well enough that your doom is sealed. you have just time to leave a sign. four o'clock, hastings. number four, the destroyer. ah! an idea!"

he rushed into the other room and seized the telephone. he asked for hanwell.

"you are the asylum, yes? i understand there has been an escape to-day? what is that you say? a little moment, if you please. will you repeat that? ah! parfaitement."

he hung up the receiver, and turned to me.

"you heard, hastings? there has been no escape."

"but the man who came—the keeper?" i said.

"i wonder—i very much wonder."

"you mean—?"

"number four—the destroyer."

i gazed at poirot dumbfounded. a minute or two after, on recovering my voice, i said:—

"we shall know him again, anywhere, that's one thing. he was a man of very pronounced personality."

"was he, mon ami? i think not. he was burly and bluff and red-faced, with a thick moustache and a hoarse voice. he will be none of those things by this time, and for the rest, he has nondescript eyes, nondescript ears, and a perfect set of false teeth. identification is not such an easy matter as you seem to think. next time—"

"you think there will be a next time?" i interrupted.

poirot's face grew very grave.

"it is a duel to the death, mon ami. you and i on the one side, the big four on the other. they have won the first trick; but they have failed in their plan to get me out of the way, and in the future they have to reckon with hercule poirot!"