The word "inch" in this paragraph has the meaning of slipping smoothly, moving in continuous motion without hindrance. A synonym that would fit perfectly and maintain the consistency of the paragraph would be the word "Creep".
Eliezer soon discovers that the chimneys are pumping out the smoke from burning the bodies of the people that have been killed at the concentration camp. The smell coming from the chimneys is unknown at first, but soon they discover that it is the smell of burning flesh.
