Ventilation of Rooms, S^c. 



119 



which, to produce the greatest effect, should vary with the state 

 of the fire : whether it is just kindled, has fresh coal thrown 

 on it, or whether it burns clear and with a red heat ; and these 

 cases have recently been met by the contrivance of a moveable 

 plate inclined forwards, so as to reach within about three inches 

 of the front of the chimney, but capable of being pushed back 

 against the further wall. To afford the means of a still 

 more accurate adjustment; a small moveable plate is fre- 

 quently inserted towards the lower part of the large one. 



It seems clear that the section of the flue should at the least 

 exceed these as four-fold, the average size of the aperture 

 communicating with the fire-place, but that at the top it should 

 be much contracted. All descent of cold air into the chimney 

 will then be prevented, which, by reducing the temperature, 



