He was 25 years old when he was enrolled (2).
He was living in Philadelphia, Pennsylvania, when he was enrolled (2).
He was enrolled and mustered into service on 27 November 1861 (1, 2 [16 Dec 61]). He was enrolled at Philadelphia, Pennsylvania (2). He was mustered in as a private, at Philadelphia, Pennsylvania (2). He served as a private in company B (1).
He deserted on 7 January 1862 (1). (According to (2), he was mustered out on 27 November 1861!)
1 Bates, Samuel Penniman. History of Pennsylvania volunteers, 1861-5. Harrisburg: B. Singerly, state printer, 1869-71. 5 volumes. 'Ninety-first regiment', volume 3, pages 186-233. (In the roster)
2 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 29 June 2004 (William Yerkes)
[I did not find an entry for him in Ancestry's index to the pension index by name (searched April 2007)]