The book is fiction inspired by the historical reality. Every biography & and every history book is speculative, nobody knows exactly what the described persons really have thought & felt and how exactly the reported events have happened. And the authors are bound to use sources which might be highly subjective and are often biased. But Kehlmann`s assumptions are plausible and convincing, the protagonists may indeed have thought as the author describes.
The protagonists didn´ t have much in common. Humboldt liked to travel and took many risks, including dangerous rides on tropical rivers, walks through dense jungles and climbs on mountains to altitudes nobody went before. He "examined everything that lacked the feet and the fear to run away from him" (except women, whom he ignored). The geeky Gauss liked the opposite sex, but disliked traveling and avoided risks.
I was entertained & amused how Kehlmann portrayed Humboldt´s adventurous journeys to South America & across Russia and the more mundane episodes in the life of Gauss, even though the novel has some bizarre & fantastic anecdotes which couldn´t have happened - maybe a trick to remind the reader of the fictional character of the book. And I learned a lot about how both men contributed to the ascent of science and the evolution of thinking.
Kehlmann shares the analytical thinking and scientific curiosity with his protagonists and he shows a lot humor. "Measuring .." is a successful mixture of entertainment and historical lecture and owns the elegance & literary ambitions that distinguish strong biographies.