56 GREENHOUSE, CONSTRUCTION AND HEATING. 
is shown at 4; but where an ornamental appearance is 
desired either of the others may be chosen. The form shown 
at Fig. 41 is Tucker’s patent anti-drip roof bar, designed 
for use in orchid houses, etc. The 3in. by Ijin. size of a 
pattern will safely carry a roof of even 2loz. glass up 
to 7ft. or Tit. 6in. in length (of rafters) even without a 
purline, and up to 9ft. or 1Oft. with this aid. In exposed 
or windy localities it is, however, safer and better to employ 
the 3in. by 2in. size, which will easily carry the heaviest 
roof up to 15ft. or so, with a purline, or to 10ft. or so 
without, the bars not being more than 16in. to 18in. apart. 
It is, however, always the best and safest plan to put 
a purline and standards in, however stout the bars may 
be, certainly where the rafters are long; but if principals 
are employed a purline is seldom necessary. 
As regards the various methods of constructing the roof, 
sufficiently full detailed instructions have already been given 
on pp. 17 and 18, so that very little remains to be said upon 
that part of the subject here. Houses constructed entirely, 
or principally, of framed lights or sashes, possess one rather 
important advantage over those built of bars and glass in 
the ordinary way, in that should the necessity for removing 
them ever arise, they are easily taken apart and recon- 
structed, while, in pulling the others down, a good deal of 
glass is almost sure to get broken, and the ends of the bars 
have generally to be re-cut before erecting again. Con- 
sequently, houses of the more portable type are really more 
valuable than the others. 
When cutting the ends of the rafters or sashbars, much 
the best method is to use a mitre-box, made beforehand to 
the right angles, one for the top and one for the bottom. 
