Page:Popular Science Monthly Volume 11.djvu/224

212 then also the state of the air was anything but desirable. It was only with 2,120 cubic feet that the medical and other authorities found themselves satisfied.

At present the demands for ventilation in France sound very different from what they were about twenty years ago. They are now per hour and person:

Such are the changes of times.

Now the many crevices, holes, and pores in our dwellings will no longer be considered by you as unlimited means of change of the air, since you know how large that change has to be; you will rather feel anxious whence to procure such enormous quantities when you sit quietly within your four walls where you do not feel the least draught, where no curtain moves, and a feather lies quietly on the floor. This sensation of calm we owe to the insensibility of our nerves—and yet the air moves.

In order to give you some idea of the influences of differences of temperatm-e of more or less well-shutting doors and windows, of a fire in a stove opening into the room, and of the partial opening of a window, I will give you shortly the results I obtained with the aid of the carbonic-acid measurement. The room had brick walls, and its size was 2,650 cubic feet.

With a difference of temperature of 34° Fahr. (66° in-and 32° out-side), the contents of the room changed once in one hour, equal to 2,650 cubic feet.

With the same difference, but a good fire in the stove, whose communication with the chimney was made as free as possible, the change of the air rose to 3,320 cubic feet, or about twenty-five per cent. When all openings, crevices in windows and doors, were thoroughly pasted up, there was still a change of 1,060 cubic feet per hour, or a fall of twenty-eight per cent. With a difference of temperature of 71° in-and 64° outside, the change amounted to 780 cubic feet only per hour. When opening a window of eight square feet, the change rose to 1,060 cubic feet per hour. These quantities are instructive. They show that a difference of temperature of 34 with carefully-shut openings