Note 29 — Chapter VI
Reading edition
In using telescopes within doors, care should generally be taken, that there be no fires in the apartment where they are placed for observation, and that the air within be nearly of the same temperature as the air of the surrounding atmosphere; for if the room be filled with heated air, when the windows are opened, there will be a current of cold air rushing in, and of heated air rushing out, which will produce such an undulation and tremulous motion, as will prevent any celestial object from being distinctly seen.