seven o'clock
Out to the front the ping ! ping ! of the rifles has become rapid and incessant , and by broad daylight all the river bank and the walls of the buildings that command a view of it are packed with gray riflemen ready for work the instant those bridge-heads loom into view . When seven o'clock comes , and the fog thins just a little , there are the bridge-ends , sure enough , poking drearily into space , but the only signs of the builders are the motionless forms in blue that are stretched here and there about the boats or planks , only faintly visible through the mist ; the working parties have been forced to give it up . Back they come , what is left of them , and tell their tale among the sympathizing blue overcoats in the wearying ranks , and officers ride away up the slopes , and there are moments of suspense and question , and then the thud of sponge-staff and rammer among the batteries , and a sudden flash and roar , tearing the mists asunder ; another , another ; and then , up and down along the line of heights , the order goes , and gun after gun belches forth its charge of shot and shell , and back from the walls of Fredericksburg comes the direful echo and the crash of falling roof or gable . `` Depress those muzzles ! '' is the growling order .