Classifications. E — FIXED CONSTRUCTIONS; E21 — EARTH DRILLING; MINING; E21B — EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS; E21B21/00 — Methods or apparatus for flushing boreholes, e.g. by use of exhaust air from motor; …