Mointor formerly used in a Clay mine
A Monitor is a type of water cannon used for hydraulic mining of soft sedimentary material by blasting it with water which then forms a slurry which is collected in ponds and screened to reclaim the minerals or metals that are being mined for. The sludgy waste and contaminated water can cause problems.
Further information: Hydraulic mining
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the term is also used for the related device used for fire fighting - Fire monitor.