The 1901 Factory and Works Act established precise limits as to the amount of water vapor that would be allowed in British industrial settings. These limits were especially important in coal mines where some water could lessen the chance of explosions, but too much water could lead to medical problems for the miners.
This hygrometer was made to measure humidity in mines, and may have been used in West Virginia. The inscription reads: "John Davis & Son (Derby) Ltd. No. 443 Derby & London. Reg No. 518758." This was an English firm that specialized in mathematical and other instruments for the mining industry.
Ref.: Henry Davis, "The Application of the Hygrometer in Coal Mines," Transactions of the Institute of Mining Engineers 35 (1907-1908): 285-290.
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