62 GREENHOUSE CONSTRUCTION AND HEATING. 
rafters of 7{t. or less in length, no purlines are necessary, 
that is, if the bars are of not less than 3in. by 1din. stuff. 
The neatest and best of all materials for a purline is 
1din. or 1din. angle-iron, with a hole drilled opposite each 
rafter, and a liin. screw put in. This obstructs scarcely 
any light, and is at once neat and very strong. It may be 
supported by means of either wooden or iron standards, 
but the latter are preferable, as being lighter and more 
durable. The only objection to this form of purline is the 
extra cost, which is much more than that of wood. 
Lengths of 38in. by 3in. quartering, ripped down 
diagonally (or corner-wise), as shown in Fig. 49, are 
frequently employed. These are easily nailed 
to the bars, top and bottom, and present a flat, 
or nearly flat, surface below to take the top of 
the standard. But, though more troublesome 
to fix, having to be ‘‘ skew nailed” to the rafters, 
ordinary 3in. by 2in. quartering (planed, of 
Fig. 50. course), is somewhat better on the whole, 
obstructing less light, and being more easily jointed or 
halved together. In this case, if wooden standards are 
employed—these should be of 3in. by 3in. stuff, or 4in. 
by 3in., for large structures—they need only be notched, 
or ‘‘bird’s mouthed,” as a carpenter would say, at the 
top, to receive the purline. Standards made of Itin. 
or 1iin. gas-barrel are frequently employed, and are both 
neat and strong. These had better have a thread run on 
each end, and a plain collar screwed on the bottom, to be 
built into a brick pier at the bottom, or fixed on a bed of 
concrete, with another collar drilled with three or four holes 
to screw it to the purline on the top. The lengths of piping 
