This proposition naturally applies also to human habitations. The air will be better in a small family house than in large barracks; better in a cellular than in a common prison, where the day and night wards are large but crowded.
The question arises, "What is to be done in all cases in which the natural ventilation of the inhabited spaces proves insufficient, and allows the carbonic acid to become more than one per 1,000?" I might tell you now of the different systems of ventilation, the contrivances and apparatus belonging to them; but this is not feasible without models and designs. And, after all, there would be no new principles or natural laws to acquaint you with. I believe I have made you sufficiently aware of the fundamental facts and conditions as to the change of the air in our dwellings, so far even that you are now able to judge for yourselves whether a certain plan for ventilation is rational or not. We have no other motors for changing the air, but differences of temperature and motion of the air, which we can call forth either by heat or by the motion of wind-fans—or which we must make use of as far as they are preexisting in the atmosphere surrounding the house. By these two means we can produce certain perturbations in the equilibrium of the air-columns, and through this certain degrees of velocity in the motion of the air.
If we know the transverse section of the inlets and outlets, we have only to multiply their surface by the' velocity of the air, and this will give the cubic quantity of the air which flows through the channels in a certain time. If we know the required quantity of air and divide it by the transverse section of the channels, we get at the velocity of the air in the channels. We ought not to establish a greater velocity than nine feet per second; it is better to enlarge the channels. These quantities must then be compared with the air required by each person, a quantity with which you are now acquainted.
If you take up the question of artificial ventilation in its quantitative aspect, you protect yourself at once against a series of errors into which else you easily fall. Our ordinary dwelling-houses need not be ventilated artificially; we ought never so to crowd them that the natural means of ventilation, as difference of temperature, motion of the air in the open, dry and porous walls, and temporary assistance by the architectural openings, are insufficient to keep undeteriorated what is most essential for our health. With these means there must go hand-in-hand the greatest cleanliness in all parts of the house, and abstention from all superfluous and avoidable pollution of the air of the house.
Before concluding, I am desirous of considering with you an expression which is in general use, but the frequent cause of wrong views about the change of the air. I mean the word draught. All kinds of complaints are habitually ascribed to it, and the danger of