2o8 THE POPULAR SCIENCE MONTHLY. 



ings during summer, because the difference of pressure is greater. 

 When, in winter, we stay in an unheated room, whose temperature is 

 only slightly above that of the outer air, ventilation is quite as weak 

 as in summer ; the air, if the windows remain closed, becomes quite 

 as bad by our presence, and we ought to air the room as in summer; 

 but we do not, because we want to protect ourselves against the 

 outside cold. The dwellings of the lower classes present frequently, 

 during the greater part of winter, this form of defective ventilation, 

 which gets worse with the length of the cold season. In the be- 

 ginning the walls are still dry and porous, and assist the ventila- 

 tion, so far as the wind helps them to do so ; but in proportion as 

 they get colder, they increasingly condense water from the air of the 

 house, and finally become so choked up with it that they allow no air 

 to pass, as you have seen in our moistened piece of mortar. Bad 

 doors and windows, unmended window-panes, remain often the only 

 routes of ventilation. Poor people, in complaining of them, are not 

 aware that they are the smaller of many evils, and a defect without 

 which they might suffer still more. 



Many of you, on hearing this, may be gratified by an unexpected 

 personal satisfaction. Those who try to alleviate the poor man's win- 

 ter by gifts of fuel not only procure for him the benefit of a warm 

 room, but also of a better and purer air in this room. You may con- 

 sider this as a scientific parable, showing that in each benevolent 

 action there lies a further blessing, even if we had not intended it. 



It follows, from these fundamental principles of ventilation, that 

 a great mistake is sometimes made in large dormitories. In the morn- 

 ing the custom is to open the windows, and to let them remain open 

 all day long, to be closed only just before bedtime. The poor sleepers 

 fancy that they are sleeping all night in a pure air. Whoever has oc- 

 casion to enter such a place in the morning, before rising-hour, starts 

 back before this "pure aii'," which had only been renewed during the 

 night partially and accidentally, and is so loaded with all kinds of 

 animal emanations that it presses with all its power on the fresh 

 comer. If there is no suflicient difference of temperature between 

 outside and inside, a partial opening of the windows during a winter 

 night is just as necessary as during a summer night, as far as regards 

 the change of the air. 



The bodies of the sleepers are certainly a small source of heat, 

 and such large sleeping-places become somewhat warmed by the hu- 

 man heat flowing from the beds, but they can never be warmed through 

 and through, so that the walls could become warmer. The water- 

 vapor exhaled by the sleepers condenses against the walls, and goes 

 on obstructing their pores till morning. A part of this water ni.'],y 

 evaporate during the time that the windows are kept open, but it will 

 be only a part, and hence the frequent breaking out of damp spots in 

 such dormitories in the course of the winter. 



