Let us remember the definition of a simile to help us try and identify one in this chapter. A simile is an example of figurative language that compares one thing with something else, normally something that we wouldn't normally relate to the first thing. The simile's function is therefore to provide us with an often surprising comparison that can help us to see objects and characters in new and novel ways. The presence of a simile is indicated by the use of the word "like" or "as" to compare, as in the sentence, "He is as brave as a lion."
If we have a look at the end of this chapter, after Diggory rings the bell, we can see that the sound that occurs after this momentous event is described using a simile:
Then at last it began to be mixed with another sound, a vague, disastrous noise which sounded first like the roar of a distant train, and then like the crash of a falling tree.
Note the two similes that are indicated by the word "like." The sound is compared at first to the roar of a distant train and then to the crash of a falling tree. Both of these comparisons indicate the suitably ominous nature of what is happening.