He was 29 years old when he was enrolled (4).
He was enrolled and mustered into service as a private on 7 December 1861, at Philadelphia, Pennsylvania (1, 4). He served as a private in company B (1).
According to Bates, he deserted on 19 August 1862 (1, 4).
On 28 July 1863, he was reported as having been dropped because he was missing in the campaign (probably after 11 June 1863) (2).
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) (William Rider)
2 consolidated morning report, 91st PA, 28 July 1863 (Private Rider [?])
3 company B, descriptive roll, entry 35 (William Righter) (no information)
4 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 31 May 2004 (William Rider)
[I did not find an entry for him in the index on <www.ancestry.com> to the pension index by name (searched April 2007)]