And next morning, rising early, as was his custom, he descended to the ground floor by means of the stairs instead of the lift, and as he passed the door from which he had seen Mrs.Marlow emerge he mentally registered the number.
Fifty-three.

Number fifty-three.
Allerdyke, who could not exist without fresh air and exercise, went for a stroll before breakfast when he was in London--he usually chose the Embankment, as being the nearest convenient open space, and thither he now repaired, thinking things over.

There were many new features of this affair to think about, but the one of the previous night now occupied his thoughts to the exclusion of the others.

What was this woman doing, coming--with evident secrecy--out of one set of rooms, and entering another at that late hour?
He wanted to know--he must find out--and he would find out with ease,--and indirectly, from Fullaway.
Fullaway always took his breakfast at a certain table in a certain corner of the coffee-room at the hotel; there Allerdyke had sometimes joined him.

He found the American there, steadily eating, when he returned from his walk, and he dropped into a chair at his side with a casual remark about the fine morning.
"Didn't set eyes on you last night at all," he went on, as he picked up his napkin.