Page:Popular Science Monthly Volume 11.djvu/220

208 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 beginning the walls are still dry and porous, and assist the ventilation, 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 winter 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 consider 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 morning 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 occasion to enter such a place in the morning, before rising-hour, starts back before this "pure air," 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 sufficient 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 human 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 may 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.