plumming

Plumming is the surveying operation used to locate the precise point on the surface where a mine should sink a vertical shaft or horizontal adit to connect with known underground workings. The surveyor uses a mine dial, a specialized compass fitted with sighting vanes and often a clinometer, to project the line of an existing lode or drift upward or downward through solid rock. The goal is to find the economical intersection point, accounting for the slope and direction the ore body actually travels as it descends or extends underground.

The mine dial itself is the essential tool, typically a brass instrument roughly 4 to 5 inches in diameter, marked with compass points and often a vertical scale. The surveyor stands at a known point within the mine, sights along the lode or tunnel using the dial's vanes, records the bearing and angle, then calculates where that line will emerge or where it should be intercepted from above. In steep terrain or deep workings, this calculation determines whether a new shaft will hit the lode efficiently or miss it entirely, wasting thousands of pounds in sinking costs.

Plumming also serves to anticipate the angle at which a lode inclines as the mine goes deeper. A vein that runs gently downward to the north will move laterally as depth increases; a surveyor must account for this drift to position future shafts correctly. Without plumming, a miner might sink a shaft only to find the lode has shifted beyond reach, or conversely, position it so inefficiently that ore extraction becomes uneconomical.

The term itself comes from the older sense of plumb, meaning to sound depths or to measure vertical alignment using a plumb bob. Though the mine dial incorporates a compass rather than a weight on a string, the principle of determining true vertical and lateral position remains. Historic mining records in Cornwall and Wales frequently mention plumming as a prerequisite before expensive sinking work began.

Plumming remains relevant wherever underground work must connect to surface or where multiple levels of workings must be coordinated. Modern mining uses transit levels and theodolites rather than mine dials, but the fundamental task, remains unchanged: project known subsurface geometry onto the surface, or predict underground geometry from surface observations, to guide the next excavation.