1. She establishes a shared challenge: She begins by acknowledging that the group (which includes Percy) had agreed to write a ghost story. This sets the stage for her later critique without directly attacking her husband's abilities.
2. She introduces the concept of a "new" type of ghost story: She suggests that their initial attempts were "dreadful" and "too horrible" because they relied on typical supernatural elements. This hints at a need for a different approach.
3. She subtly elevates her own idea: She then proposes the "interesting" concept of a "being" born from "the instruments of science" instead of the traditional ghostly apparitions. This positions her own story as a more thought-provoking and innovative take on the genre.
4. She shifts the focus away from her husband: By highlighting the shared failure of the group and then presenting her unique idea, Mary Shelley implicitly acknowledges Percy's involvement without singling him out for criticism.
In short, Mary Shelley uses clever wording to gently suggest that the traditional ghost story might be limited without directly criticizing Percy's abilities. She emphasizes the need for a new approach, paving the way for her own innovative creation, Frankenstein.