2 14 THE POPULAR SCIENCE MONTHLY. 



This proposition naturally applies also to human habitations. The 

 air will be better in a small family house than in large barracks ; bet- 

 ter 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 diflferent systems of ventilation, the contriv- 

 ances and apparatus belonging to them ; but this is not feasible with- 

 out models and designs. And, after all, there would be no new prin- 

 ciples 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 surround- 

 ing the house. By these two means we can produce certain perturba- 

 tions in the equilibrium of the aii'-colurnns, 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 ac- 

 quainted. 



If you take up the question of artificial ventilation in its quanti- 

 tative 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 ex- 

 pression 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 



