192 



FIXED STRUCTURES FOR GROWING 



direction parallel to the pediments, but oblique to it. This is done to pre- 

 vent the water from running down on one side of the glass, which it would 

 do in consequence of the general slope of the ridge from the back to the 



Fig. 133. Vertical profile of part of a ridge and furrow roof. 



front if the bars were placed at right angles to the ridge. The angle at 

 which the bars are fixed will vary with that formed by the slope of the ridge, 

 and the mode of determining it, is to place the bars so that the lap of the glass, 

 which is in square panes, may form, when the panes are fitted in their places, 

 lines truly horizontal. There are many persons, however, who attach no 

 great importance to causing the water to run down the middle of the glass 

 instead of one side ; and they will, of course, place the bars for holding the 

 glass, parallel to the pediments, in order to avoid the short bars at the ends 

 of the ridges, as seen in fig. 133. For more minute details respecting this 

 mode of construction, we refer to Paoetoris Magazine of Botany, vol. ii. 

 p. 80; and Gard. Mag. vol. xx. p. 452, and also for 1841. 



485. The materials used in the construction of plant-houses differ in 

 nothing from those used in other buildings, excepting that where as much 

 light as possible is required to be admitted, the framework for containing the 

 glass is formed of iron or other metal, as supplying the requisite strength 

 with less bulk than wood. The proportion of opaque surface of an iron 

 roof may be estimated at not more than 7 or 8 per cent., while in a wooden 

 roof it is upwards of 20 per cent. ; both roofs being in one plane and of the 

 ordinary construction (279 and 281.) Where sheet-glass is employed, and 

 the panes made of more than ordinary length and width, as in the large 

 conservatory recently erected in the Horticultural Society's garden, the pro- 

 portion of light admitted in the case of iron roofs will be found still greater. 

 Ridge and furrow roofs, if we take the area of the bases of the ridges as the 

 total area of the roof, and then deduct from it the space occupied by the 

 bars forming the sides of the ridges, and the ridge-pieces and gutters, will 

 not appear to admit the same proportion of light as a roof in one plane ; 

 but the practical result will be different, in consequence of the sun's rays 

 being twice in the day perpendicular to one half of the roof, the advantage 

 of which to the plants will far more than compensate for the obscuration 

 produced by the greater proportion of sash-bars, which operating chiefly at 

 mid-day and in very hot weather, is rather an advantage than otherwise. 

 To prove this, it is necessary first to know the law of the reflection of light 

 from glass. 



48G. The law of the reflection of light from glass was calculated by 

 Bouguer, a French philosopher, in 1729, and is exhibited by the following 

 figures ; the" first line representing the angles of incidence, and the second 

 the number of rays reflected, exclusive of decimal parts. 



