I ceased to pursue a train of thought which promised to lead to morbid depths, and directed my attention to what Smith was saying.

“We will drop down from Wapping and reconnoiter, as you say the place is close to the riverside.  Then you can put us ashore somewhere below.  Ryman can keep the launch close to the back of the premises, and your fellows will be hanging about near the front, near enough to hear the whistle.”

“Yes,” assented Weymouth; “I’ve arranged for that.  If you are suspected, you shall give the alarm?”

“I don’t know,” said Smith thoughtfully.  “Even in that event I might wait awhile.”

“Don’t wait too long,” advised the Inspector.  “We shouldn’t be much wiser if your next appearance was on the end of a grapnel, somewhere down Greenwich Reach, with half your fingers missing.”

The cab pulled up outside the river police depot, and Smith and I entered without delay, four shabby-looking fellows who had been seated in the office springing up to salute the Inspector, who followed us in.

“Guthrie and Lisle,” he said briskly, “get along and find a dark corner which commands the door of Singapore Charlie’s off the old Highway.  You look the dirtiest of the troupe, Guthrie; you might drop asleep on the pavement, and Lisle can argue with you about getting home.  Don’t move till you hear the whistle inside or have my orders, and note everybody that goes in and comes out.  You other two belong to this division?”

The C.I.D. men having departed, the remaining pair saluted again.

“Well, you’re on special duty to-night.  You’ve been prompt, but don’t stick your chests out so much.  Do you know of a back way to Shen-Yan’s?”

The men looked at one another, and both shook their heads.

“There’s an empty shop nearly opposite, sir,” replied one of them. 
“I know a broken window at the back where we could climb in. 
Then we could get through to the front and watch from there.”

“Good!” cried the Inspector.  “See you are not spotted, though; and if you hear the whistle, don’t mind doing a bit of damage, but be inside Shen-Yan’s like lightning.  Otherwise, wait for orders.”

Inspector Ryman came in, glancing at the clock.

“Launch is waiting,” he said.

“Right,” replied Smith thoughtfully.  “I am half afraid, though, that the recent alarms may have scared our quarry—­your man, Mason, and then Cadby.  Against which we have that, so far as he is likely to know, there has been no clew pointing to this opium den.  Remember, he thinks Cadby’s notes are destroyed.”

“The whole business is an utter mystery to me,” confessed Ryman.  “I’m told that there’s some dangerous Chinese devil hiding somewhere in London, and that you expect to find him at Shen-Yan’s.  Supposing he uses that place, which is possible, how do you know he’s there to-night?”

“I don’t,” said Smith; “but it is the first clew we have had pointing to one of his haunts, and time means precious lives where Dr. Fu-Manchu is concerned.”