Explanation:

 Bhalku joined Harrington’s team and soon became the most important man in it. His only instrument was a long and solid wooden staff with which he would hit a section of the mountain wall inside the tunnel and listen to the sound. He repeated the exercise on various points inside the tunnel and then filtered the information based on the sound. When he found something, which he invariable did, he would instruct the engineers to dig from the point he’d zeroed upon. And they followed him.