In Chapter 8, Poole arrives at Utterson’s door in a state of high anxiety and terror, believing that Hyde has harmed Jekyll. As Utterson and Poole hurry back to Jekyll’s house, the moon is described as hanging sideways in the sky:
It was a wild, cold, seasonable night of March, with a pale moon, lying on her back as though the wind had tilted her [...].
This simile lends an atmosphere of chaos, changeability, and reversal to the night of Jekyll’s death. Even those elements that seem most fixed, like the moon in the sky, move as though affected by an unnatural force (here, an impossibly strong wind). The agency given to the moon, as she “l[ies] on her back,” is also off-putting. The moon follows the rules of nature. But on this particular night, “she”—the personified moon—has suddenly been given a new and threatening autonomy, threatening because it suggests that the typical laws of the natural world are open suddenly and without warning to change or manipulation. As it helps build an environment of urban chaos and dread, this simile feeds into the Gothic atmosphere that reigns throughout the novella.
This simile also reflects certain elements of Hyde’s arc over the course of the novella. Hyde, like the moon, was previously tethered by the laws of nature, and thus relegated to a secondary position within Jekyll’s ego. Under the influence of the supernatural experiments undertaken by Jekyll, though, Hyde gradually frees himself from his previous confinement. On the night this simile describes, that unshackling of Hyde from Jekyll’s consciousness is finally complete, as Jekyll transforms for the last time.
In his final letter to Utterson, Jekyll uses a simile to describe the potency of the ingredients in the potion he takes to become Hyde.
Certain agents I have found to have the power to shake and pluck back that fleshly vestment, as a wind might toss the curtains of a pavilion.
This line conveys both how powerful Jekyll’s potion is and how easily Jekyll believes he can manipulate the laws of nature. Ironically, Jekyll's simile likens the supernatural work of the potion to the strength of a natural agent of change: the wind. He also compares the physical body to something man-made (“curtains” hung over a pavilion). The hubris of Jekyll’s experiment is already clear in the strange loftiness of its goal: he wants to be the first scientist to successfully tear a metaphysical hole in the human personality, separating the “good” and “evil” of his own interiority. But with this simile, Jekyll’s ego comes to the surface even more, as he portrays this task as impossibly easy and inevitable. Jekyll's goal is as easily accomplished by his problem-solving as a curtain is stirred by the winds.
This simile reveals something about Jekyll’s broader view of humanity as well. He doesn’t speak of “his” body, but refers more generally to “that” body (“fleshly vestment”), which holds the interiority of any human being. His comparison of the potion to a "natural" force (the wind) and the body to a flimsy, unnatural barrier (curtains) implies that the separation of our socially acceptable "good" sides from our darker natures is artificial or illusory.
Jekyll thinks everyone has a dark side, which is just barely hidden from view (as a curtain obscures the particulars, but not the shape, of what’s behind it). This implication is disturbing, given that Jekyll’s “dark side” as manifested in Hyde is cruel, sadistic, and violent. Jekyll’s conclusion is undercut by the horror other characters feel upon seeing Hyde or hearing of his crimes. This simile thus points to a level of serious delusion in Jekyll.