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548 physical study that he could not resist, and he began again, where he had left off in former years, with the principle of the transformation and conservation of energy and the dynamical theory of heat. The question of the cause of the Glacial epoch was much discussed among geologists. Without knowing what Herschel and Lyell had written upon the matter, he conceived the change in the eccentricity of the earth's orbit as probably the real cause, and began in 1864 a series of papers of great importance on the subject, setting forth the solutions "which make his name one of the most illustrious in the history" of theoretic geology. They were published in the Philosophical Magazine and the Reader, and were elaborated in the course of ten years in the book Climate and Time. These papers are characterized by Mr. James Campbell Irons, Croll's biographer, as "distinguished by remarkable concentration of thought, joined to a very great lucidity of exposition," and are considered by him in detail in six groups, of which those on Geological Climate and Chronology, Glacial Epoch and Glaciers, and Ocean Currents include the most weighty contributions.

The papers as they appeared attracted the attention of men of science at home and abroad. The Geological Society of Glasgow in 1867 elected their author an honorary associate, and Prof. Alexander Ramsay, chief of the Geological Survey, and Dr. Archibald Geikie, director of the Scottish department of the survey, were so struck by them that Mr. Croll was offered a position in the Scottish service, to be resident surveyor and clerk in the office at Edinburgh. He was well satisfied with his position in the Andersonian College and reluctant to leave it, but besides a larger salary this place offered some other advantages over that, and its duties promised to leave him as much time and strength to accomplish the work of investigation on which he was engaged as the one at Glasgow. He was obliged to submit to a civil-service examination. His knowledge having been acquired in a life of work and not in the formal routine of school, and he being very nervous, he failed on questions of arithmetic, and in English composition. Dr. Geikie nevertheless insisted on having him, knowing his value, and was supported by other eminent geologists; and at last the Lords of the Civil List, in consideration of many special recommendations in his favor and much labor on the part of his friends, were induced, as Lord Kelvin has it, to accept Croll's "great calculations regarding the eccentricity of the earth's orbit and the precession of the equinoxes during the last ten million years as sufficient evidence of his arithmetical capacity, and his book on The Philosophy of Theism and numerous papers published in the scientific journals as proof of his ability to write good English," and he received the appointment. The duties of