312 WAEMING AND VENTILATING OCCUPIED BUILDINGS. 



where the air is contaminated by injurious exhalations, in order to pre- 

 vent them from affecting the air of the rooms. Inversely, fresh air 

 should be introduced at points removed from the occupants. 



3. The different plans which act by means of a draught, when they 

 are properly proportioned and well made, fulfill better and more surely 

 the foregoing conditions than those which act exclusively by forcing in 

 fresh air. The latter do not, of themselves alone, secure the removal 

 of foul air uniformly and constantly under all circumstances and in all 

 seasons. 



4. The introduction of fresh air taken at the desired height and in 

 sufficient quantity may be obtained in most cases by the action of 

 draught alone, and without the help of blowing-apparatus, by making 

 the air-shafts and their openings sufficiently large, and having them 

 properly arranged. 



5. The draught may be produced, first, by the fire-places or stoves 

 with their chimneys, which are used for general heating or by similar 

 apparatus; secondly, by the same means, aided, if necessary, by auxihary 

 fires at the bottom of ventilating-shafts, 50 to 65 feet high in large 

 establishments, when these are needed. The air to be removed should 

 flow toward the base of these sliafts; in most cases it should be carried 

 there through one or more channels which branch out and terminate in 

 openings close to the sources of infection. 



6. Ventilation, by means of draughts produced by grates and chim- 

 neys, is easily adapted in most cases to all the modifications rendered 

 necessary by the size and arrangement of rooms. It approaches, as 

 closely as could be desired, the usual and natural aeration of rooms and 

 apartments; it allows the amount and temi^erature of the air-currents 

 to be varied as needed. It only requires cheap fire-places and their 

 chimneys and pipes or channels, which, once made, cost little to keep in 

 order. It needs no other attention than the regular supply of fuel to 

 the fires, to which account all the attendance may be charged. 



Ventilation by blowing or by mechanical apparatus requires, besides 

 chimneys and ventilatiug-flues, common to both systems, blowing- 

 engines and steam-engines, with special channels for the introduction of 

 the air-blast. It requires the attention of special laborers, mechanics, 

 and firemen, besides involving expense for repairs. 



7. For hospitals or for buildings having several floors, the blowing- 

 system does not give the same guarantees as the other system against 

 carrying foul air from one room to another, nor against the return of 

 foul air through the air-shafts or through cracks in their sides, when an 

 accidental circumstance, as the opening of doors or windows, disturbs 

 the usual pressure and motion of the air in the rooms. 



8. Draught maintained by simple fire-places and chimneys, with open- 

 ings of sufficient size suitably placed for the admission of fresh air, 

 carries off" the foul air without the help of any mechanical apparatus, 

 and becomes, except under exceptional circumstances, the readiest 



