18 GREENHOUSE CONSTRUCTION AND HEATING. 
to 12in. apart, and supported in the middle, in the case of 
long roofs, by one or more cross-bars of angle or other iron. 
These last are secured to the principals at each end, and 
drilled and screwed to each bar, and may be fixed in one, 
two, or even three places, according to the length of the 
rafters. If sashes are used, some of the upper ones— 
usually those in each alternate ‘‘bay’’—are fitted so as to 
slide, like a window-sash, and are pulled up and down by 
means of cords and pulleys, and thus used as ventilators 
(see p. 35). 
When preparing to erect a greenhouse of any magnitude, 
the first thing, after deciding upon its dimensions, etc., is to 
make a set of drawings to scale, including a ground plan, 
elevation and cross section. If this is carefully done, going 
into the details as far as possible, it will be found of great 
assistance in working, as the pitch and many other im- 
portant points, as well as the size and position of the doors, 
ventilators, etc., can be obtained from such plans almost at a 
glance. 
In designing a horticultural building of any kind, the 
chief point to be considered is the purpose for which it is 
intended. As already stated, one class of plants is found to 
succeed best in a certain type of house, and others in 
another, so that where, as usual, the highest degree of 
excellence is desired, it is most important that each and 
every structure be specially designed and adapted for the 
particular subject or purpose in view. Certain plants, as 
for instance the tomato, rose, camellia, and others, always 
thrive best in very light and airy houses, preferably of con- 
siderable dimensions and abundantly ventilated. Cucumbers, 
on the other hand, require very little ventilation, and usually 
