153 



FIXED STRUCTURES FOR GROWING 



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. 131. For more minute details respecting this mode of construc- 



Fig. 131. 



Vertical profile of part of a ridge and furrow roof. 



tion, we refer to ' Paxton's Magazine of Botany,' vol. ii. p. 30 ; and 

 * Gard. Mag.,' vol. xv. p. 452, and also for 1841. The Great Exhi- 

 bition Building of 1851 in Hyde Park, with the Crystal Palace at 

 Sydenham, furnish magnificent examples of this mode of glazing. 



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

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

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

 taining 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. 

 Where sheet- glass is employed, and the panes made of more than 

 ordinary length and width, as in the large conservatory erected 

 in the Eoyal Horticultural Society's garden, the proportion 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 conse- 

 quence 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 propor- 

 tion of sash-bars, which, operating chiefly at mid-day and in very 

 hot weather, is rather a.n advantage than otherwise. To prove this, it 

 is necessary first to know the law of the reflection of light from glass. 



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 : 



Angle of incidence . . . 85, 80, 70, 60, 50, 40, 30, 20, 10, 1. 

 Percentage of rays reflected 50, 41, 22, 11, 5, 3, 2, 2, 2, 2. 



