thirl

To thirl is to drill or bore horizontally through solid rock, typically to connect two separate underground workings or to break through a pillar that separates them. In coal and hard rock mining, this was essential work: when adjacent seams or panels needed communication, or when a barrier had to be breached to allow passage of air, water, or personnel, a thirl had to be cut. The term belongs to the vocabulary of older mining practice, now largely displaced by more precise modern terminology and mechanical methods.

Historically, a thirl was bored by hand drill or, later, by compressed air drill, using operators working in confined spaces. The work was directional and required accurate alignment: a miscalculated thirl might miss the target working entirely, or breach an unintended partition and flood the mine. The hole itself might be small (to serve as an air passage or probe) or larger, depending on what needed to pass through. Hand-thirl operations demanded skill and physical endurance, often in damp or gaseous conditions.

The word and its context

The verb thirl comes from Old English and Scots mining dialect, related to the idea of piercing or perforating. It survives in Scottish and Northern English mining records and technical reports from the 18th and 19th centuries. The corresponding noun, thirl, referred both to the act and the resulting opening. Related terms include thrall (to bore) and thirl-stone (rock drilled through or alongside).

Thirling was particularly common in coal mines where pillars separated adjacent panels, and where controlled ventilation required communication holes. In metal mines, thirling served to access new ground or to relieve pressure on pillars. The practice declined as underground drilling became faster, safer, and more mechanized, and as mining engineering moved toward planned layouts that reduced the need for improvised connections.