X, a mill owner, employed independent contractors to construct a reservoir for his mill. During the course of construction the contractors came upon some old shafts and passages on the land of X. The contractors did not block them. When the reservior was filed, water burst through the old shafts and flooded the mines of 'Y'. 'Y' sued 'X'. 'X' was held liable. He could have not been liable if he could show that