The knocking begins very softly.
At the foot of the stairs the match went out, and he paused to strike another; and at the same moment a knock, so quiet and stealthy as to be scarcely audible, sounded on the front door.
At first Mr. White tries to ignore it. But the increasingly louder knocking attracts his wife, who is eager to open the door. Mr. White is terrified. He knows what Herbert must look like after being shredded in a factory machine and then lying in a grave for some time. The word "fusillade" is appropriate in the context because it not only suggests a tremendous series of blows on the door, but it augments the sense of danger. A fusillade is a term used in warfare. It is defined as a series of shots in quick succession or all at once. A fusillade could be rifle shots or cannon shots. In this case it would seem to sound more like a fusillade of cannon-fire. Whoever--or whatever--is doing all that knocking is not to be denied entrance, even if it means battering the door down. And the word "fusillade" suggests that he may be angry and violent. Mr. White's only recourse is to find the monkey's paw and use it for his third wish, which would obviously be to make that "thing" go away. The word "fusillade" is used only once, and very effectively, in Part III of the story.
If he could only find it before the thing outside got in. A perfect fusillade of knocks reverberated through the house, and he heard the scraping of a chair as his wife put it down in the passage against the door.
Evidently the author calls it a "perfect" fusillade because it sounds so much like a real fusillade. Of course, the reader will never know for sure that it really was the mangled Herbert knocking or whether it might have been some lost traveler who was desperate for help and knew the Whites were at home because he had seen Mrs. White standing in the bedroom window holding a candle. As Mr. White told his son Herbert early in Part II:
"Morris said the things happened so naturally," said his father, "that you might if you so wished attribute it to coincidence."
It could have been a coincidence that Herbert got killed at the factory the day after his father wished for 200 pounds. It could have been a coincidence that the amount of money the factory owners decided to award the Whites was the same as the amount wished for. It could have been a coincidence that someone other than Herbert came knocking at the door after Mrs. White had persuaded her husband to wish for him to return. It could even have been a coincidence that the road was deserted when they finally opened the door. Whoever was doing the knocking could have given up and left. That stranger might not have been seen on the road because he was tramping across an open field. On the other hand...it might have been Herbert!