you could live in a city all your life , and you had to go away and come back again before you noticed the way the rain dripping off the gutters had a rhythm all its own : dumdi-dim-dim , dumdi-dumdidum-dum ... a few minutes later sergeant colon and corporal nobbs of the night watch were sharing a friendly roll-up in the shelter of a doorway and doing what the night watch was best at , which was keeping warm and dry and staying out of trouble .