A trapper was a child employed to open and close doors in roadways along which the coal tubs were transported. The practice of employing boys to do this work continued after the 1842 Act Mines and Collieries Act prohibited all girls and boys under ten years old from working underground in coal mines.

Reference:
Mining Occupations, Durham Mining Museum website http://www.dmm.org.uk/educate/mineocc.htm

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