166 LECTURES. 



groups .of apartments and of individual rooms that required the power 

 of independent action in a more comprehensive scheme, that would 

 economize and facilitate the whole operation, without sacrificing the 

 special requirements of each separate control ? 



These preliminaries being settled, the next step was to determine 

 whether a ventilating shaft^ put in action by heat, should be resorted 

 to for the necessary power, or a mechanical instrument sustained by 

 a steam engine or any equivalent force. 



Where offices occupied by a few individuals only were to be venti- 

 lated, and where they were only required for very brief periods, neither 

 large shafts nor machinery might be requisite, if proper apertures 

 for the ingress and egress of air were arranged, as in well-ventilated 

 individual habitations, with small ventilating shafts or flues. 



A shaft being made to operate on the vitiated air to be discharged, 

 tended, more or less, to produce a comparative vacuum in the apart- 

 ment to be ventilated, and hence the origin of the term Vacuum ven- 

 tilation. 



An instrument moved by mechanical power, and acting directly in 

 expelling vitiated air, produced a similar effect. But when it was 

 made to ventilate by blowing in fresh air, it tended to create an excess 

 of pressure within the apartment it ventilated ; air then escaped out- 

 wardly by open doors and windows, as well as by any appointed 

 channels, if they were not extremely large. This was termed Plenum 

 ventilation. 



In the most perfect form of ventilation, the ingress and egress of 

 air were so nearly balanced that there was little or no tendency to 

 the air being drawn inwards or pressed outwards at doors or other 

 apertures not provided for its regular ingress or egress. The less 

 the tendency to either plenum or vacuum ventilation the better.^ And 

 even where shafts alone, or instruments alone, were used, it was 

 always desirable to reduce the tendency to a plenum or vacuum as much 

 as possible by the right adjustment of supply and discharge. In law 

 courts, theatres, or assembly rooms of great complexity, and having 

 numerous entrances to galleries, to seats on the floor, and to special 

 places allotted for particular purposes, and still more if they were 

 subject to great fluctuations of attendance, a plenum and vacuum 

 power was combined where the greatest perfection of effect was desired. 

 Having determined on the leading arrangements for the supply and 

 discharge of air, the amount to be given per minute, the apparatus 

 required for heating, cooling and moistening, and any of those end- 

 less varieties of contingencies which each individual building might 

 require, whether from the purposes to which it was to be applied, the 

 locality in which it was to be placed, or the climate to which it was 

 subject,— the details of the supply and discharge, the position of 

 valves, and the precise arrangements required for the ingress and 

 egress of air, should then be planned. This, in general, will be found 

 to require much more attention than was formerly given to such 

 questions. It is the rock of difficulties in all disputes where separate 

 authorities are responsible for decoration and structure, and for the 

 comfortable and effective result of ventilation. If the architect do not 

 profess ventilation, or the authorities do not confide that department 

 to him, it will be obvious that if no right mutual understanding be 



