308 WARMING AND VENTILATION. 



.,..,.. t. .. r 13 ^- 3x1130000 

 To renew the air three times an hour, it should carry off ........ 



=942 cubic feet a second. The velocity vrhich the solar heat may give to 

 the escaping air being estimated at but If feet a second, the sectional are''^ 

 of the veutilating-space should be 574 square feet ; and if the ventilating- 

 passage is carried the whole length of the roof, which is 328 feet, it 

 would suffice to make it 1 foot 9 inches wide. But as the part where there 

 is most smoke and steam is usually near the end at which the trains leave, 

 instead of making the ventilating-opeuing extend the whole length of 

 the station, it would be better to give it greater breadth and less length, 

 still retaining the same sectional area. 



COURTS AND COVEEED MARKETS. 



100. Similar arrangements should be adopted in the case of courts 

 and for all covered markets. 



In the latter, where blinds are usually placed in the windows, the 

 introduction of air is easily provided for, and it is particularly- the 

 remov^al of foul air that requires attention. 



GLASS ROOFS AND CEILINGS. 



101. Injluence of glazed roofs and ceilings during the 2vinter, — If in the 

 summer season the glazed roofs of stations and covered courts present 

 the inconvenience of producing a heating effect, which it is necessary 

 to overcome, in winter they have the contrary defect, which often leads 

 to very disagreeable results. 



The conductibility of thin glass then leads to a considerable cooling 

 of the interior layers of air in contact with the glass ; this air, becoming 

 denser than that below, descends, and is constantly replaced by more, 

 which is likewise cooled, and by this continued movement the rooms 

 thus covered become very difficult to warm. 



To these troubles is added that of the motion of the cold air, which 

 naturally flows toward the chimneys, or the discharge-openings, if there 

 are any, so that the occupants feel a descending current of cold air, the 

 more unpleasant the nearer they are to the chimney or the discharge- 

 openings. 



If the glass roof is simple, and has, as is almost inevitably the case, 

 joints, through which the external air — much colder than that in con- 

 tact with the internal surface — penetrates into the room, the effects 

 which have been mentioned become more sensible and disagreeable. 

 There is also the danger that water will enter during rain-storms. 



It is, then, necessary in occupied buiUlings, when similar plans are 

 adopted for lighting, to place under the roof a glass ceiling with as few 

 joints as possible, and in the loft thus formed and limited above and 

 below, to provide heating arrangements which will prevent the cooling 

 of the ceiling, and thus to avoid the cold air currents which have just 

 been referred to. 



