PITS AND FRAMES. 125 
the roller up, with the blind rolled on it, to the top. A 
long open-sided box, or pent-house, is fixed along near 
the ridge to receive the blind, and protect it from the 
weather (see Fig 80). 
There can be no doubt of this being decidedly the best 
system of working blinds on greenhouse roofs. The details, 
such as the arrangement of the pulleys and working cords, 
may be varied to some extent, but the principle remains. 
But even by this method the rollers must be neither very 
heavy nor too long, or the blind will soon give way. A 
hollow metal tube— 
preferably of tin or 
brass—is superior to 
wood in its greater 
rigidity and lightness, 
and lengths of 20ft. 
or 25ft. should not be 
exceeded, even with 
this material. 
When the house . Rid: 1. 
does not exceed the above in length, a single roller 
may be employed, with a good sized pulley at each 
end, which should overhang the ends of the structure 
slightly. In this case the blind may rest, and the rollers 
run, right down on the bars or rafters, but where more 
than one length of roller is required the blinds and rollers 
must be kept up off the roof by means of iron rods fixed 
as shown in Fig. 81. Without these the pulleys could 
-not work, or even be fixed in place, and beyond this it 
is found that better results are obtained when the blind ig 
elevated a little way above the glass. 
