A Question Concerning Dew on Coach-Glasses WHEN two or three persons are in a coach, and draw up the glasses, it is observed they become so covered with dew, that we can scarce see through them; but when that is once wiped off, there is no more dew gathered upon the glass, but it continues transparent. You will oblige your readers with the reason of this phenomenon. THE ANswer. THE reason is, that in comparison of the moist vapours that come from the persons in the coach, the glass is cold, and condenses them, remaining cold longer than any other part of the coach; as we find in damp weather, that marble will become wet by condensing the moisture of the air. Then by degrees, the glass, partaking of the warmth of the persons in the coach, is no longer able to condense the floating vapours into water. The proof of this is plain by letting down the glass into its place, because there it cools, and then being brought up it again condenses the vapour and gathers a dew; without which it would not condense the vapour, though in many hours' travelling.