54 8 POPULAR SCIENCE MONTHLY. 



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 impor- 

 tance 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. Alex- 

 ander Eamsay, 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 Scot- 

 tish service, to be resident surveyor and clerk in the office at 

 Edinburgh. He was well satisfied with his position in the An- 

 dersonian 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 accom- 

 plish the work of investigation on which he was engaged as the 

 one at Glasgow. He was obliged to submit to a civil- service ex- 

 amination. His knowledge having been acquired in a life of work 

 and not in the formal routine of school, and he being very nerv- 

 ous, he failed on questions of arithmetic, and in English composi- 

 tion. 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 pub- 

 lished in the scientific journals as proof of his ability to write 

 good English," and he received the appointment. The duties of 



