RING

is the art of penetrating rocks, etc., in the formation of artesian wells, ——_ eS alte a baw Aaiae: |;; soil-pipes are driven in @ ground unti the bard rock is reached and then drilling tools are employed. The cutting tool, known as a bit, is attached to a combination of jointed rods by screw connections, and at various pane contrivances known as “* jars” aro introduced for the purpose of le ening the jar on the rod at each downward stroke, ‘The whole is attached by a — A) eae at ——— which is raised and depre ed by engine power. In boring modern tunnels, the harder kinds of rock have been penctrated by means of a shield furnished with diamonds as cutting instruments. The shield is pre ed against the face of the rock and made to revolve. The rock is worn away but, owing to their extreme hardne , the diamonds are not even scratched. In tunnelling under rivers, to oh ee the works being flooded, the device has been adopted of working tho shield in a comp res wed air chamber. In the event of water being tapped, tho pre ure of the air is sufficient to keep it back until the tubes have been placed in position, and the space between them and the rock has been filled in with concrete. [s. 77]