358 THE AIR OF TOWNS. 



second, it finds its way through the chinks round the door or between 

 the window sashes. It naturally follows that where cold air is enter- 

 ing' through small inlets to supply 200 cubic feet a minute, drafts are 

 frequently experienced by persons in the room, unless mechanical con- 

 trivances are arranged for directing the cold air to the toi> of the room. 



It follows that ventilation produced by the currents set up by warm 

 air is closely connected with the methods of warming a room. Begarded 

 from this point of view, the open fireplace is the reverse of economical. 

 The whole of the heating is here produced by radiation ; that is, by heat 

 passing from the fireplace to the walls, ceiling, and floor, which in turn 

 transfer their warmth to the air in contact with them, and this repre- 

 sents a small fraction of the heat passing up the chimney. 



A more economical method is to warm the air of rooms by means of 

 steam or hot-water pipes; but in this case there is no natural ventila- 

 tion, no fresh air is introduced as with the open fireplace, and special 

 means must be provided to supply the defect. 



Another method is to supply a house with fresh air, which has been 

 slightly warmed by passing it around a stove fixed in the basement or 

 out of doors. " In this case, if a suitable exit is provided to permit the 

 vitiated air to escape, a constant current. of fresh air is set up, which 

 may effect the whole heating and ventilation of an ordinary dwelling 

 house at a comparatively small cost for fuel. In large buildings, such 

 as warehouses and factories, the same result is effected by pumping in 

 at the basement fresh air, warmed bypassing through a stove and mixed 

 in any desired proportion with cold air and drawing off the vitiated 

 air by means of an exhaust fan placed at the top of the building. 

 These principles may be demonstrated by the following experiments: 



The illustration (fig. 12) represents a shallow, air-tight box with a 

 glass front. Three small circular holes are bored along one side equi- 

 distant and one at the bottom of the opposite side. In A this hole is 

 fitted with a glass T piece, the top vertical end of which passes through 

 a cork of a lamp chimney. Through the same cork a gas burner is 

 fitted. The box is filled with a dense fog by blowing in ammonium 

 chloride fumes and is brightly illuminated by a lantern. When the gas 

 jet in the chimney is burning, one of the circular holes is opened to 

 the air, and the lower vertical end of the T piece closed, we have on 

 a small scale the conditions of ventilation in a room with an open fire- 

 place. The air enters through one or all of the circular holes, appearing 

 in the fog like black smoke, and the white fumes -are observed to issue 

 from the top of the lamp chimney. The other experiment figured at 

 B is to illustrate heating and ventilation by warm air. Air enters the 

 box through the bent pipe, which is heated by a burner. The warm air, 

 which appears at the top of the foggy chamber as a dark cloud, grad- 

 ually displaces the fog, which is driven out at the lower left-hand aper- 

 ture and the chamber is thus filled with warm fresh air. 



The importance of placing within the reach of every person a method 

 of determining quickly and accurately the amount of carbonic acid in 

 the air has induced me to devise a process, a description of which will 

 be found in the apx^endix. 



