a considerable degree of Heat will be quickly excited in those parts of the iron where the File passes to and fro, the many prominent parts of the Instrument giving a multitude of strokes or pushes to the parts of the iron that happen to stand in their way, and thereby making them put the neighbouring parts into a brisk and confus'd motion, and so into a state of Heat. Nor can it be well objected, that upon this account the File it self ought to grow as hot as the iron, which yet it will not do; since, to omit other answers, the whole body of the File being moved to and fro, the same parts, that touch the iron this moment, pass off the next, and besides have leasure to cool themselves by communicating their newly received Agitation to the air before they are brought to grate again upon the iron, which, being supposed to be held immoveable, receives almost perpetual shakes in the same place.
We
