It's a sign I'm a bit slow, but I just realized that several of the articles in the 1911 Catholic Encyclopedia, which is online, are by none other than Pierre Duhem himself. The articles are:
History of Physics
Pierre de Maricourt
Jordanus de Nemore
Albert of Saxony
Thierry of Freburg
Jean de Sax
This should have occurred to me before; after all, Duhem -- physicist, historian of science, philosopher of science, Catholic -- would have been the perfect person at the time to do articles like these. So there are several articles online on the history of science, particularly medieval science, by the greatest historian of science of the twentieth century. That's quite a nice thing to find out. I especially recommend the "History of Physics" article, which gives a Duhemian perspective on physics up to late nineteenth century thermodynamics, in a very concise and readable form.
Duhem, of course, was a thermodynamicist, one of France's very best; his name is attached to the Gibbs-Duhem relation and the Clausius-Duhem inequality because he took the work of Gibbs and Clausius and made it more rigorous. He began working early on the history of mechanics, which led him to do groundbreaking research into the medieval roots of science. One of the mysteries of his career is why France's most brilliant mind in thermodynamics was assigned to a low-level teaching position at an insignificant school in the French school system, where he was isolated from the main work done at Paris and had few resources to work with. Stanley Jaki has argued, with some degree of plausibility, that it was all politics -- Duhem had embarrassed the wrong people, and they made sure to dampen his career as much as they could. Whatever the reason, Duhem's work in history of science and philosophy of science alone would be enough to earn him a place among the intellectual giants of the past few centuries.
You can also read his Physics of a Believer online, as well as excerpts from his masterpiece in philosophy of science (and here) and his masterpiece in history of science, thanks to Joseph Barrett. Readers of French can enjoy parts of Le système du monde at Gallica (scroll down to Duhem here for other parts).
[ADDED LATER: Additional online versions of Duhem's texts can be found in several formats here, thanks to Alain Blachair.]