SYSTEMS OF VENTILATION 329 



located was rarely overheated. That room was certainly 

 supplied with an abundance of fresh air. Great volumes of 

 air swept up the wide, open-throated chimney, to be replaced 

 by fresh air which crept in through the numerous cracks 

 between the logs or around the loosely fitted windows and 

 doors. This was all in marked contrast to our modern houses 

 with their nearly air-tight walls, heated by means of stoves or 

 radiators, often with no provision whatever for the entrance of 

 fresh air or the exit of foul air. It is a fact worth noting that 

 although the colonists did suffer greatly with the cold they did 

 not suffer as we do today with colds and pneumonia. 



362. Modern Systems of Ventilation. Even the humblest 

 dwelling, heated by means of stoves, may be fairly well ven- 

 tilated by being flushed out at frequent intervals by throwing 

 open doors or windows. Besides this flushing method, there 

 are several systems of ventilation in more or less common use 

 all of which are based upon the constant dilution of the vitiated 

 air. They all fall into one or the other of two classes ; NATURAL, 

 or GRAVITY SYSTEM, and the FORCED SYSTEM. 



SYSTEMS OF VENTILATION 



1. Natural or Gravity System. 2. Forced Systems. 



(a) By means of doors and (a) Air moved by propulsion, 



windows. "Plenum System." 



(6) By means of special air (6) Air moved by suction. 



' shafts. ' (c) Combination of propulsion 



Generally used in private resi- and suction, 



dences and small buildings. Generally used in large public 



buildings. 



363. Ventilation of Dwellings. The system of ventilation 

 employed in dwelling houses is largely determined by the 

 method of heating employed. When heated by means of 

 stoves the dwelling is usually ventilated either (1) by frequent 

 flushings or (2) by window ventilation. The latter is a form 

 of the gravity system. The cooler, outside air is permitted to 

 flow in at the bottom of the window and force the warmer, 



