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 Statistics of American Soldiers," published by the United States Sanitary Commission, and in the same year he graduated with distinction from the Scientific School, receiving the degree of B. S. While at the university he also went over an important part of the classical course.

With a view of continuing his mathematical studies, he secured an appointment to the Military Academy at West Point through the Hon. J. W. McClurg, Representative of the Fifth Missouri District in Congress, and was admitted as a cadet to the Academy on the 1st of September, 1866. On the 15th of June, 1870, he was graduated, third in a class of fifty-nine members, and was appointed Second-Lieutenant of the Fourth United States Artillery. He was assigned to Company G of that regiment, and served with his command at Fort Johnston, North Carolina, until August, 1871, when he was ordered to duty at the Military Academy, West Point, as Assistant Professor of Natural and Experimental Philosophy. June 10, 1872, he was transferred to the Engineer Corps of the army, remaining at West Point, however, as instructor in practical military engineering.

During these few years at the Military Academy he published, in the "American Journal of Science," his first astronomical work—a short paper on the "Spectrum of the Aurora" and another on the "Spectrum of Lightning."

In March, 1873, Lieutenant Holden resigned his commission in the army, and was appointed Professor of Mathematics in the United States Navy, a position which his teacher Chauvenet had held before him. He was ordered to the Washington Naval Observatory, then under the direction of Rear-Admiral B. F. Sands, and his first duties in his new profession were as assistant to Professor Harkness in the work of the transit circle. Immediately, however, upon the completion and mounting of the twenty-six-inch equatorial—at that time the largest refractor in the world—he was transferred, November 15, 1873, to duty as assistant to Professor Newcomb, who had been placed in charge of that instrument.

The results of Professor Holden's work during six years of observation with this instrument, first as assistant to Professor Newcomb, and afterward as assistant to Professor Hall, have been printed in the volumes of the Observatory publications and in various astronomical journals. Turning over the files of the "Astronomische Nachrichten" for these years, we find numerous observations of comets and of double stars, of the satellites of Uranus and Neptune, and of the companion of Sirius; but, besides all this rather prosaic routine work. Professor Holden devoted himself zealously to the more fascinating study of the physical features of the planets and nebulæ. His most elaborate investigation in this field is given in the