VENTILATION, 31 
narrow lean-to houses against high walls, intended for the 
culture of peaches, vines, or tomatoes, &., a pitch of 50 tc 
60 degrees, or even more, may be advantageously adopted, 
as affording an extra length of rafter. Such steep roofs also 
catch the full force of the sun when ee 
this is low in the sky, in the winter and REY, 
early spring, when it would be reflected 
to a greater extent from flatter ones. 
(See Fig. 17). 
The length of the rafter is in all 
cases equal to the secant of the angle 
or pitch. 
The Venti- § a 
lation,— This y/ Wane 
is another point yy *, 
requiring care- 
°C 
2 VA : 
: / rf a 
ful considera- Z Je 4 
tion and proper fe | 
provision. aK 
Unless the Vi, 
internal at- B 
. £1 | 
mosphere of a il 
greenhouse of ia a 
any kind is eae TRE 
J Coe oS 
'G 
at any rate __ mV : 
. ae PuiluB~ —~ ee Sy < 
occasionally 
Fig. 17 
changed, the 
health of the plants it contains will probably suffer, sooner 
or later, though as already stated certain classes of plants 
require much more ‘‘air’”’ than others. Such plants 
as pelargoniums (geraniums), primulas, marguerites, and 
