Harper's Weekly was a prominent periodical published in New York City. As an 'embedded artist,' Mosler developed his drawings in the field, sometimes on the battlefield. He sketched the troops and landscapes. Later he conveyed the materials to John Bonner, his editor, hundreds of miles away. Mosler explained this process matter of factly in a diary entry from October 8, 1862.
In the morning I made a sketch of the Battle at Perryville and the town of Perryville in about 3 hours and sent them to be mailed at Louisville by our sutler of the 9th Indiana
A sutler was a civilian merchant who toured with the troops and sold provisions such as tobacco, sugar, and coffee. This sutler may have worked as a go-between for Mosler when he was in remote locations and unable to mail items directly himself.
Once the drawings arrived in New York, other artists transferred Mosler's images to wood engravings.