Matross was a soldier of artillery, who ranked next below a gunner.
The duty of a matross was to assist the gunners in loading, firing and sponging the guns. They were provided with firelocks, and marched with the store-wagons, acting as guards. In the American army a matross ranked as a private of artillery.
The word is probably derived from French matelot, or from German Matrose, a sailor.
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. Encyclopædia Britannica Cambridge University Press
The original article can be found at Matross and the edit history here.