CONTINUOUS VENTILATORS. 35 
now. In the cases of roofs constructed with framed 
lights and heavy rafters, it is a very simple matter to 
make the upper half of every alternate sash or bay to 
slide up and down in suitable guides, each being fitted 
with a pulley and cord. Even where the roof is 
made without sashes, the spaces between the principal 
rafters being filled in with light bars and glass, as 
described on p. 17, a sliding light may be fixed in 
every second or third bay. The disadvantages of this 
plan are that heavy rafters and a good deal of sash-work 
are required, which of course add to the expense, while the 
Fic: 20. 
sashes cannot be opened in wet weather without admitting 
the rain. 
Ventilators made the whole length of the rafters, and 
hinged at the bottom instead of the top, are occasionally 
made use of, as is also a type consisting of a long narrow 
sash, usually the width of a single roof-square only, and 
hinged at one side (Fig. 21), but none of these forms are as 
effective as the ordinary flap-ventilator hinged to the 
ridge-plank. 
Cheap, low houses intended for the growth of bedding- 
plants, carnations, and so forth, may be ventilated at 
D 2 
