"It's not rational to treat 2+2 as 5." — logical as contrasted to illogical
"I know you're upset, but please think about this in a rational manner." — based on reason as contrasted to emotion
"When I was 10-years-old, I wasn't very smart, but I was still rational." — capable of using reason as contrasted to being insane or lacking the ability to reason
"In matters of the heart, I trust my intuition more than my rational analysis." — based on a logical analysis as contrasted to intuition, instinct, custom, tarot-card reading, or some other system of decision making
In a fit of enthusiastic madness I created a rational creature and was bound towards him to assure, as far as was in my power, his happiness and well-being.
Chapter 24 (70% in)
When I reflected on the work I had completed, no less a one than the creation of a sensitive and rational animal, I could not rank myself with the herd of common projectors.
Chapter 24 (44% in)
There are no more uses of "rational" in Frankenstein.