This is an interesting question. I've never thought the duck trouser detail was anything more than Stephen Crane putting in a specific, real detail. Duck cloth is a heavy, plain-woven cotton fabric. It's sometimes used for art canvases because of its stiff, durable nature. It makes sense that Crane points out that kind of clothing detail. Those pants are cheap to produce and durable. The material is also advantageous in a number of ways for artillerymen. The material isn't prone to snagging or tearing, and those men are constantly pushing and pulling heavy metal and wood around to fire their pieces. Duck cloth is also excellent at blocking wind. These soldiers are up on hills and ridges that overlook battlefields.
On the top of the hill a battery was arguing in tremendous roars with some other guns and to the eye of the infantry, the artillerymen, the guns, the caissons, the horses, were distinctly outlined upon the blue sky.
That location is likely to be one of the colder and windier...
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