The "wondrous power" is magnetism. Mary Shelley, working within the accepted scientific
discourse of her time, in this second paragraph of her novel, is carefully establishing
a conceptual—the term in her time would have been "philosophical"—base for it in physics.
The key is provided by Percy Bysshe Shelley's citation, in the opening sentence of
the Preface (I:Pref:1), of Erasmus Darwin, to whom the reader may turn for further
elucidation of the scientific grounding of the novel.