almanac than it ever directly admits. We knew a man who, every spring, according to the almanac, used to say to his children, on the first fine day, "The back of the winter is broken; I must change my underclothing," the result of which always was that he was confined to the house with a cold for the next few days. We know plenty of people who will not light a fire until they deem that the almanac warrants their having a fire. For our part, if the weather is cold enough we have a fire in July, and if it is hot enough we fan ourselves in December. So experience and common sense, and we, as their temporary representative, alike proclaim as reasonable.

CHAPTER XXXIII.

VENTILATION WITH REFERENCE TO HEALTH.

VERY one has some notion, however inadequate as a

EVERY

proper guide to practice, of the value of fresh air; advice to take fresh air being prescribed by every one for himself and others. This general notion, however, goes but a little way toward the proper regulation of life with reference to the necessity to health of having constantly an abundant supply of fresh air, for thousands of persons who realize the hygienic value of out-door air take not the slightest precaution as to securing purity of air in their dwellings; and yet, upon an average, more than the half of life is spent in-doors.

Therefore, to possess really valuable knowledge on the subject, such as leads to establishing the habit of ventilating, it is necessary that the requirement of the body as to pure air shall be known. Pure air is just as much the food of the body as comestibles are. The breathing of persons in the closed spaces called rooms vitiates the air. Every person by breathing contributes just so much poison to it constantly. The flame of an ordinary gas-burner makes a draught upon it equal to the breathing of it by four persons. Nature takes care of the purity of out-door air, and is ready to aid us in taking care of that in-doors; but if we do not accept the proffered aid, it does not compel us to do so. It merely visits this, as an infraction. of one of the laws of health, with feebleness, sickness, and death.

An erroneous popular notion confounds warm air with impure air, and cool air with pure air. A room is not necessarily close because it is too warm, nor necessarily pure because it is cool. One of the most sickening of all smells is in the close, cold

houses of the wretchedly poor, who are constrained, in order to avoid suffering from cold, to exclude outside air as much as possible. Of course, both warm air and cool air may be impure to any degree down to foul, but, of the two kinds, the warm air is, under average conditions, likely to be the purer, because constantly tending to be purified by interchange of volume for volume with the outside fresh air, whereas the cool air remains relatively stagnant.

This last observation naturally leads at once to the inquiry as to what is the active agency in the purification of the air of houses. The determination of that question, and the best action deducible from it, make the basis of the whole art of ventilation.

To ventilate a house is not to purify the air in it, but gradually to get rid of the impure air in it by gradually replacing it with fresh air. The air in it is, as compared with outside air, always more or less impure; so our object should be so to dilute the whole intermingled volumes of good and bad air by a constantly entering volume of pure air, that the air of the house shall be virtually pure.

This purpose can be accomplished in two, and in only two, ways: by mechanical means, such as pumps and fans, and by natural means-action growing out of the difference of density of contiguous volumes of air. The mechanical means are best adapted to certain classes of buildings and to mines; the natural means are for all the world. It is only with reference to the latter that we are called upon to speak, for they represent ordinary ventilation.

Air becomes less and less dense, that is, becomes lighter and lighter, the more that it is heated. Consequently, air in houses tends to lie in what we may, for convenience, regard as a series of strata gradually growing from below warmer and warmer as they grow less and less dense in ascending to greater and greater heights.

In the winter-time, in the climate of Philadelphia, rooms are, when their occupants can afford to heat them at pleasure, at the temperature of about 70° Fah., and the outside temperature ranges from about zero to 40° Fal. We have, under these conditions, plenty of motive power at our disposal to discharge the impure air from our houses, and take in place of it all the pure air necessary for health. If we foolishly caulk up and weather-strip every perceptible aperture, we should not be surprised at producing weakly conditions, nor even at the appearance of typhoid fever. We have unlimited pure air at our disposal, and all the necessary means of moving it, both furnished by nature. If we bar nature out from acting, we do it at our own cost.

The next question that arises is as to what is the volume of the stream of air that we should discharge from our dwellings, in order to insure that the corresponding volume of outside air received shall maintain that in the reservoir of the house in a virtually pure condition. Statistics say that the lowest permissible fresh air every hour for a grown person is 1000 cubic feet. A preferable amount to this is deemed to be 2000 cubic feet per hour, and some persons advocate even 3000 cubic feet per hour. Higher amounts are needed for the sick, some sanitarians recommending as high as 3000 to 4000 cubic. feet for the sick. All these estimates are, of course, founded upon investigation of the degree to which air in a house is liable to become vitiated from breathing and other causes.

It follows, from what has been said, that the best ventilation is that which supplies ample amount of air without changing the temperature and without occasioning draughts. In turn it. follows that outside air allowed to enter a building is best injected through many sources of supply, and best ejected through many vents. The proper movement of the air there, and the consequent purity of the air, depend as much upon the

possibility of exit as of entrance. If registers of a cellar-furnace are giving out in lower rooms of a house a large volume of heated air, it is certain that an equal volume of air is in some way or other finding escape from the house. But although, as we have said, the warmer the air is, the higher it will ascend, it must not be restrained from ascending. Like everything else in motion, it goes in the lines of least resistance. If the windows in the upper stories of the house are almost hermetically sealed, and those near the register are comparatively free for the egress of air, the hot air will escape through the latter, and thus be wasted for the useful office of passing through and warming and scouring out the upper rooms.

Draughts are obviated by the simplest precautions. In the best dwelling-houses, hospitals, and factories, the ventilation is effected by means of vertical shafts in the walls, so arranged that the ingress of fresh air is near the floor and of waste air near the ceiling, but so that the registers for these different purposes shall not be exactly underneath each other. But, without these appliances, by judicious use of windows, depressing the sashes above and raising them below, with guards, if necessary, for the open space below, all draughts in a house can be prevented, while, at the same time, it may be ventilated thoroughly. In the cold winter climate of this latitude, the ventilation may be sufficiently secured by means of the cracks in the sashes, if persons would forbear from a general weatherstripping of the house. Every case is a special one, to be considered and acted upon as such. The whole question upon which to base a decision is as to the fact of how much fresh air is being received. Different kinds of building result in such differences of permeability in different houses, that no general rule can be given as to whether or not to weather-strip, and, if it be decided to weather-strip, to what degree to have it done.

In the morning, after the family have risen and left their