In "The Adventure of the Speckled Band," Holmes describes Dr. Roylott, the antagonist, using a metaphor p.32:
"When Dr. Grimesby Roylott drove past us it was with a hollow, muffled rumble, which reminded one of those black-beetles which, hurrying along with their heads down, make a dry, sharp hissing as though the very distance around them were full of snakes."
In this metaphor, Dr. Roylott's carriage is compared to a black beetle. .
In the similar the sound of the carriage is compared to the snakes hissing .
Both are used to emphasize the ominous and menacing nature of his character.