A Reciprocal Simile

On the third day, the decisive day, at Gettysburg, the opening artillery barrage of the Confederates sounded to everyone in the town like thunder -- a low and continuous rumble, but one to wake the dead.  And not just in the town; it was heard 20 miles away.  When would it end, and who would survive the onslaught?

But in the wee hours this morning, the thunder sounded like artillery.  In the beginning it seemed that there was no break in the sound, although I knew that it was made up of hundreds of thunderclaps merging in the dense air.  Only when the storm was nearly overhead did the reports become discrete and staccato, and the flashes visible, as if the cannons were Federal now and we were marching up the long incline against flanking fire of canister and grape, soon now to be awarded the Red Badge of Courage.