VENTILATION. 



207 



double function is in action, there is a certain amount of 

 impediment to movement offered by the influence of the 

 opposing currents of air on each other. Hence we find that 

 the action of a ventilating shaft is improved by separating it 

 into two divisions, each of which can act as an outlet or an 

 inlet, independently of the other (Figs. 19, 20, 21 and 22). In 

 McKinnell's method (Fig. 23), the separation is effected by 



1 



\ 



Fig. 19. 



i 



Fig. 20. 



using two concentric shafts (one within the other) ; the inner 

 one being made to project above the outer one, so as to render 

 it an outlet as much as possible. 



One of the best examples of an artificial lead to an outlet 

 for air is afforded by an ordinary fire in a room, in which 

 the chimney is maintained as an outlet by the fire causing the 



\\ / J 



Fig. 21. 



Fig. 22. 



Fig. 23. 



production of heated and consequently light gases, which 

 while escaping upwards, are followed by a current of air that 

 takes their place. In this, as in other forms of convection of 

 heat, we find that for the outlet to work well, there must be 

 free admission of air by an inlet ; for if we close up all the 



