Mr. Merryweather is the bank director. He is sure that his bank is safe and this is a wild goose chase. He even remarks that "I hope a wild goose may not be the end of our chase" Only after he is assured by the police agent that Sherlock Holmes has detective qualities, does he think this is worthwhile. To enter into the bank, they have to go through a small corridor,ending in a massive gate. The massive gate was opened by Mr. Merryweather and then he led them down a flight of winding stairs which ended up at another massive gate. Mr. Merryweather opened that one and led them further down until they got to a huge vault or cellar, which was piled all around with boxes. Holmes states that the place seems secure from above, and Mr. Merryweather assures him it is, as well as from below. They have stationed flags on the floor. He hits these with his cane and is surprised to find they have a hollow sound. Holmes gets upset. He is expecting the robbers to come through the floor. He tells Mr. Merryweather that he needs to be quiet because the noise he has already made has put the possibility of capturing the criminals in jeopardy. If they heard Mr. Merryweather, they would scrap the whole idea until a later time or permanently. He asks Mr. Merryweather to sit on one of the crates and stay out of the way. After examining the floor, Holmes is sure that they are about to capture the criminals.
On Saturday night Holmes goes to the underground bank strongroom with Watson, Mr. Merryweather, and a policeman. The banker believes the room is burglar-proof and taps on the flooring to demonstrate.
“Nor from below,” said Mr. Merryweather, striking his stick upon the flags which lined the floor. “Why, dear me, it sounds quite hollow!” he remarked, looking up in surprise.
“I must really ask you to be a little more quiet!” said Holmes severely. “You have already imperilled the whole success of our expedition. Might I beg that you would have the goodness to sit down upon one of those boxes, and not to interfere?”
Holmes knows by now that there is a tunnel underneath the floor, and he is alarmed and angered by the noise the banker makes with his cane. The two crooks might not be underneath the flooring at this point, but the sound of the tapping could travel a long way through the tunnel. If the burglars heard such a sound it would suggest that there was someone in the strongroom and also suggest that someone must suspect the existence of a tunnel. They would undoubtedly decide to wait. But eventually the lure of the gold would draw them to the strongroom. Only now they would not be caught off guard. John Clay has a pistol, and the other man may also be armed.
The author only inserts this little incident of the tapping to add to the suspense. The reader wants to see the burglars get caught, but now he has to worry about whether the burglars will even show up. The long wait in the complete darkness and the stealthy entry of John Clay through the opening in the floor are the climax of this story. Conan Doyle takes care to create the maximum amount of suspense and excitement.
If Holmes didn't want Mr. Merryweather to "interfere" with what he and the other two men were doing, then why did he invite Merryweather along? The answer is obvious. The bank director has to let them into the room, and he is responsible for the 30,000 gold Napoleon coins stored there. Doyle had to invent such a character as Mr. Merryweather to explain how Holmes could get inside to set his trap for John Clay. The banker has to open three doors.
...following the guidance of Mr. Merryweather, we passed down a narrow passage and through a side door, which he opened for us. Within there was a small corridor, which ended in a very massive iron gate. This also was opened, and led down a flight of winding stone steps, which terminated at another formidable gate. Mr. Merryweather stopped to light a lantern, and then conducted us down a dark, earth-smelling passage, and so, after opening a third door, into a huge vault or cellar, which was piled all round with crates and massive boxes.
The presence of Mr. Jones from Scotland Yard gives official sanction to Holmes's actions. Holmes is on very good terms with the police because he has helped them solve many cases and usually allows them to take full credit for the results.