Three years before publishing the _Principles of Mining_ Hoover had collaborated with a a group of authors in the production of a book called _Economics of Mining_. And three years later, that is in 1912, he privately published, in sumptuous form, with scrupulously exact reproduction of all of its many curious old woodcuts, an English translation of Agricola's "De Re Metallica," the first great treatise on mining and metallurgy, originally published in Latin in 1556, only one hundred years after Gutenberg had printed his first book. "De Re Metallica" was the standard manual of mining and metallurgy for 180 years. Georgius Agricola, the author, was really one Georg Bauer, a German of Saxony, who, following the custom of his time used for pen-name the literal Latin equivalents of the words of his German name. This translation, with its copious added notes of editorial commentary, was the joint work of Hoover and his wife--it was Mrs. Hoover, indeed, who began it--and occupied most of their spare time, especially their evenings--and sometimes nights!--and Sundays, through nearly five years. They had been for some time collecting and delving in old books on China and the Far East and ancient treatises on early mining and metallurgical processes, and had accumulated an unusual collection of such books, ransacking the old bookshops of the world in their quest. In 1902, Mrs. Hoover while looking up some geology in the British Museum Library, stumbled again on Agricola, which she had forgotten since the days she was in Dr. Branner's laboratory. By invoking the services of one of their friends among the old book dealers the Hoovers soon owned a copy. Caught especially by the many curious and only half understandable pictures in it they began to translate bits from it here and there, especially the explanations of the pictures, and in a little while they were lost. Nothing would satisfy them short of making a complete translation. It became an obsession; it was at first their recreation; then because it went very slowly it seemed likely to become their life avocation.