A. The external air (being colder than the air of the room we occupy) rushes through the window crevices _to supply the deficiency_, caused by the escape of air up the chimney, &c. Q. _Why is there more DRAUGHT if you open the LOWER SASH of a window, than if you open the UPPER sash?_ A. If the _lower_ sash be open, the _cold external air_ will rush more freely _into the room_; but if the _upper_ sash be open the _heated air of the room_ will _rush out_; and (of course) there will be less draught. Q. _By which means is the ROOM better VENTILATED, by opening the lower or the upper sash?_ A. A room is better _ventilated_ by opening the _upper sash_; because the hot vitiated air (which always ascends towards the ceiling) _can better escape_. Q. _By which means is a HOT ROOM more quickly COOLED--By opening the upper or the lower sash?_ A. A hot room is _cooled more quickly_ by opening the _lower sash_; because the cold air can enter more freely by an _under_ current, than by one _higher up_. Q. _Why does WIND DRY damp LINEN?_ A. Because dry wind (like a dry sponge) imbibes the particles of vapour from the surface of the linen, as fast as they are formed. Q. _Which is the HOTTEST PLACE in a church, chapel, or theatre?_ A. The gallery. Q. _Why is the GALLERY of all public places HOTTER than the lower parts of the building?_ A. Because the heated air of the room _ascends_, and all the _cold air_ (which can enter through the doors and windows) _keeps to the floor_, till it has become heated. Q. _Why do PLANTS often grow out of WALLS and TOWERS?_ A. Because sometimes the _wind_ blows the seed there with the dust; and sometimes _birds_ (flying over) drop the seed which they had formerly eaten. CHAPTER XXIII. BAROMETER. Q. _What is a BAROMETER?_ A. A weather-glass, or instrument to show the changes of the weather, by marking the _variations in the weight of air_. Q. _What is a THERMOMETER?_ A. An instrument to show how _hot or cold_ anything is. Q. _What is the DIFFERENCE between a THERMOMETER and a BAROMETER?_ A. In a THERMOMETER the mercury is _sealed up from the air_: In a BAROMETER the mercury is left _exposed_ (or open) to the air. Q. _If the mercury of the thermometer be SEALED UP from the air, how can the air AFFECT it?_ A. The heat of the air passing _through the glass tube_ into the mercury, causes it to _expand more or less_, and rise in the tube accordingly. Q. _Why is the TUBE of a BAROMETER left OPEN?_ A. That the air may _press upon it_ freely; and as this pressure is _more_ or _less_, the mercury _rises or falls_ in the tube. Q. _How can weather be affected by the WEIGHT of the air?_ A. When air is warm or moist, it is _lighter_ than usual: When it is cold or dry, it is _heavier_: and as a barometer marks whether the air be _light_ or _heavy_, it indicates these _changes_. Q. _How can you tell (by looking at a BAROMETER) what KIND of WEATHER it will be?_