34 GREENHOUSE CONSTRUCTION AND HEATING. 
of glass (between the bars). See Fig. 19. For cu- 
cumber or forcing-houses, doxble the above distance 
between each ventilator will suffice, but it is at all times 
advisable to provide for plenty of ventilation rather than too 
little. 
Where a superior class of house is required, and 
a little extra expense is no object, continuous venti- 
lators along the ridge will be found decidedly prefer- 
able to those already described, as well as affording a 
much more pleasing appearance. (See Fig. 20.) These 
are constructed in much the same manner as _ the 
LZZp LIL 
SATAN 
SMM, —Wn 
ANN 
LLP SAUNA MMM VANNTES 
Fig. 19, 
others, but are framed together in long lengths of 
12ft. to 20ft. or so, and 18in. or 2ft. in width. They 
may be rebated together at the ends, to prevent any 
leakage, if desired, and if fttted with proper gearing 
can be opened or closed along the whole of one side 
of the house, however long, at once. Of course, these should 
be fitted along both sides of the ridge in whole and 
three-quarter span houses. 
Years ago, sliding roof-ventilators were very generally 
employed, though they appear to be almost out of date 
