The shafts are usually 4 feet square, but, where an extensive plant of pumping machinery is required, the size is increased to 8 feet by 6 feet.
By means of two plumb-lines, the connection between the underground- and surface-surveys has been effected with con- siderable success by Mr. E. Clark* in the brown hæmatite mines belonging to the Glendon Iron Co. of Pennsylvania.  The shafts are usually 4 feet square, but, where an extensive plant of pumping machinery is required, the size is increased to 8 feet by 6 feet.  The depth of the shafts varies from 75 to 200 feet.