Sentence:  The "Mines" are cold where the outward Air comes in; but where not, warm. The greateſt trouble they have is by duſt, which ſpoileth their "Lungs" and "Stomachs", and <WSD>frets</WSD>  their "Skins". 
Meaning of the word marked with <WSD> tags:  To gnaw; to consume, to eat away.