"It's Isaac Newton's PhilosophiƦ Naturalis Principia Mathematica," he said simply. "It's considered one of the most important works of physics and mathematics ever written. It provided a great deal of the scientific background for the Industrial Revolution and is often considered to be the beginnings of the modern field of calculus."
He looked over at her, a bit concerned. He might not be a father, but he was very fond of children. When JJ had been pregnant he had kept an eye out for her, and had learned that while pregnant women weren't as delicate or breakable as they sometimes seemed, their comfort was sometimes very elusive.