Never Vie for a Viscount by Regina Scott

The perky Lydia Villers is determined to leave behind her life as a social butterfly and do what she’s always dreamed of doing, pursue a career in natural philosophy. A shame the only scientist available to assist her is the one man she had once hoped to wed.
The brilliant Frederick, Viscount Worthington, Worth to his friends, has been betrayed too many times, including by the bubbly beauty who now wants to work at his side. How can he believe Lydia’s intentions are true this time? After all, someone is intent of sabotaging his efforts to create the perfect hot air balloon. But with the help of Miss Thorn and her beloved cat Fortune, the enthusiastic young lady and the wary lord might just discover that only together do they make the perfect chemistry.