PI-AKT-HOUSES. 



483 



and this is highly essential in every situation. In houses 20 or 30 feet long, 

 and from 10 to 15 feet broad, the sashes of the roof should be made to slide, 

 so that the upper ones may be let down at pleasure, and the lower ones drawn 

 up; or they may be hinged at the upper end, and made to lift up. In addi- 

 tion to this, the upright sashes should either be made to open outwards, by 

 being hinged at one side (which, in general, is the best mode) or at the top ; 

 or they may slide in two grooves, so that the one can be pushed past the 

 other. These modes are applicable to green-houses 20 or 30 feet in length ; 

 but, for those under 20 feet in length, it will generally be sufficient to have 

 one or more small openings in the roof or sides. It must, however, never be 

 forgotten that the atmospheric air forms an important part of the food of 

 plants ; and that unless they have a sufficient quantity, they become sickly, 

 and will not produce either fruit or flowers. One of the simplest and most 

 elegant modes of having an opening in the roof is by having a cap to be 

 raised by means of a vertical rod, with a line and pulley, as shown in Jig. 

 357. In this figure, which is a cross section of a span roof, a represents the 

 cap ; b, the rod by which it is raised ; c, a cross piece of iron, in which the 

 rod b works ; d, the cord passing over the pulley e, for raising and lowering 

 the rod; and/, the rafter (forming part of the span roof) which supports the 

 whole. The rod and bars of the cap are generally of iron ; and the panes of 

 glass small, to lessen the risk of breakage. Sometimes the cap is balanced 

 by a weight attached to a cord which passes over a pulley fixed on the other 

 side of the rod opposite to ej and which, by counterbalancing the greater 

 part of the weight, and leaving little more than the friction by the rod pass- 

 ing through the bar c and the rafter / to 

 be overcome, renders it easy to raise a 

 cap of the largest size, either by hand, 

 or by a self-acting apparatus to be here- 

 after mentioned. Though we have shown 

 in Jiff. 357. this cap on the ridge of a 

 span roof, yet it may be constructed 

 with equal ease on the upper part of the 

 slope of any pent roof, or even in any 

 part of that slope, by introducing on the 



upper side of it a cross bar, or a flashing of lead, to throw off" the rain to 

 the two sides. Air may also be admitted through the side sashes, by hinging 

 a flap in the upper part of any of the sashes, with a lever and pulley to open 

 it, as shown in Jig. 358. In this figure ^r is a lever, 

 which, when pulled by the cord h, raises the flap at 

 pleasure to any point not beyond i. Whenever venti- 

 lators of this kind are used, they ought to be placed 

 in the highest part of the house ; because as the hot- 

 test air always ascends, it consequently collects there, 

 and will pass off by any opening with greater rapidity 

 than it would through an opening on a lower level, 

 admitting a counter current of fresh air to supply its 

 place. 



531. When the sloping sashes of a span roof are 

 made to slide, the operation of opening them may be 

 greatly facilitated by balancing them with a weight in 



2 I 2 



358 



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