Thanks! I knew about the hemp bails, but didn't realize they were soaked in water.
The Union soldiers did something similar during the siege of Vicksburg. They stacked bales on a rail car and move it forward, digging their entrenchments from behind its relative safety. It approached nearer and nearer every day. Finally, one Confederate figured out that if he put turpentine soaked cotton into the base of a minie ball, it might just light the bales. Shots were fired and after a while, some smoldering was seen. Word spread and the Confederates poured fire into it with the result being the bales were beginning to burn. Any Union soldier who attempted to extinguish the flames was shot at or shot down. Flames consumed the car, forcing the Union soldiers to abandon it.