The Force of Nature and The Perplex'd Duchess are collected in a compilation called Four Novels of Eliza Haywood. However, as I have already read Lasselia and The Injur'd Husband, the other two novels that make up the four, this review will only be of the first and last novel in the collection.
The Force of Nature, or the Lucky Disappointment (1724) is a novel of misunderstandings, confusions, missed opportunities, and coincidences. It follows the fortunes of a young couple in love. They have grown up together as close as brother and sister, as Fernando has been taken under the guardianship of Alvario, Felisinda's father. Of course, they fall in love, and Alvario decides to separate the young lovers. What follows is rather implausible but rather entertaining. There's mishaps in convents, duels, disguises - all kinds of melodrama. Another couple is introduced as a foil to Felisinda and Fernando. The ending is a little disturbing to a modern reader, but it genuinely surprised me, which is an unusual feat for an early 18th-century novel! Typical Haywoodesque fun.
The Perplex'd Duchess, or Treachery Rewarded (1727) is a short novel about an evil woman. Gigantilla, in spite of her rather unfortunate name, is a typical scheming Haywood villainess, clever and shrewd and manipulative. She's remarkably good at what she does, rising from the servant class to become the Duchess of Malfy - the equivalent of a queen. Of course, Gigantilla is undone eventually, but it is only by chance that her schemes are thwarted. I love reading about Haywood's bad girls, because although their schemes are eventually thwarted, Haywood obviously has a great deal of fun showing women bending gender conventions and assuming power in their own right. This is a really short novel - I read it in one sitting - but it's hugely fun, in typical Haywood style.