32 GREENHOUSE CONSTRUCTION AND HEATING. 
other ‘cool-house” subjects, as well as tomatoes, roses, 
the entire race of what are termed ‘‘ hard-wooded ” plants— 
i.e., camellias, azaleas, heaths, epacris, and so forth—need 
to be freely ventilated at almost all seasons of the year, 
while on the other hand, cucumbers, with palms and many 
other tropical or ‘‘stove” plants, rather enjoy a confined 
atmosphere than otherwise, and structures intended for the 
culture of anything of the kind do not require to be provided 
with so many or so large ventilators as houses constructed 
for any of the other subjects mentioned above. It is always 
advisable, notwithstanding, to provide for a moderate 
amount of ventilation, as any house of any kind may of 
course be required at some future time for a different 
purpose, and it is at all times as well to have the means of 
admitting air freely if necessary. 
Houses constructed in the old-fashioned style, with 
numerous small squares of glass and consequently 
innumerable ‘laps,’ do not as a rule require nearly so 
much ventilation as structures of the modern type, built 
with larger squares and few laps, and comparatively 
speaking as ‘‘tight as wax “when closed. In the former, 
an almost constant current of air is passing through the 
numerous laps, more particularly in windy weather, and the 
necessity for artificial ventilation is of course considerably 
reduced thereby, whilst in very close or ‘‘tight’’ houses, 
the ventilators must be carefully managed if sudden and 
violent changes in the temperature and atmospheric moisture 
are to be avoided. 
Two sets of ventilators are usually provided, one placed 
along the ridge or apex of the roof, and another along the 
front or sides of the house. In the case of a lean-to 
