The Corps of Invalids was a separate branch of the Continental Army. It was composed of Continental Army veterans who had become unfit for field duty but who could still usefully serve as guards for magazines, hospitals, and similar installations. The noncommissioned officers were also supposed to be proficient in mathematics because the corps was intended to serve as a military academy in addition to its other duties. The military academy seems to have been a dead letter, but otherwise the corps carried out its duties, most notably at West Point, New York.
Source(s): http://en.wikipedia.org/wiki/List_of_Continental_Army_units_(1777–1784)