GARDEN STRUCTURES. 



203 



the ridge-and-furrow roof is suitable where less 

 intensity of solar heat is required, and in cases 

 where, in a lean-to and even in a span-roofed 

 house, the plants would be too distant from the 

 glass. It should, however, be observed that 

 ridge-and-furrow roofs are things of the past, 

 unless a series of span -roof forcing houses, 

 joined together at the eaves, can be called ridge- 

 and-furrow. 



Curvilinear Roofs (fig. 264). — The principal 

 advantage which these possess is, that they can 

 be constructed so as to admit more light than 

 those which are on one plane. The light ad- 

 mitted is in proportion to the relative quantities 

 of opaque and transparent materials used : and 

 in a straight roof the area of surface occupied 

 by wooden materials is much greater than when 

 iron is employed. Again, the curvilinear iron 



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Fig. 263.— Section of Unequal Span-roofed House. 



roof can be made lighter than a straight one of 

 the same material ; because, in the curved form, 

 in order to support steadily the same weight, 

 the bars do not require to be so thick as when 

 they are straight. The curvilinear form is that 

 by which the greatest amount of light can be 

 transmitted; and in this respect it must be con- 

 sidered the most advantageous. On the other 

 hand, it must be admitted, that whilst roofs of 

 this description admit solar light and heat in 

 greater quantity than where wood is used, yet 

 they have the disadvantage of transmitting heat 

 more rapidly from the interior in consequence 

 of radiation taking place from a larger surface 

 of glass than in a straight-roofed house of the 

 same size, and the rapid cooling of the iron 

 employed in their construction. 



In constructing curvilinear roofs, it is desirable 

 that they should form some segment of a sphere, 



and not that of a spheroid; and the question is, 

 What segment is the best 1 On referring to fig. 

 264 it will be observed, that if the whole quad- 

 rant a b c were taken as the form, the glass near 

 the base at a would be nearly perpendicular; 

 whilst that at the top b would be almost flat — too 

 much so for the rain to pass off readily. Another 

 form was, however, proposed by Mr. Knight, and 

 this we consider to be unobjectionable. It is 

 represented by d e /, and is obtained as follows : 

 With a radius of 25 feet describe the curve a b ; 

 from the base cut off 35°, and from the summit 

 15°, as from b to e. This gives a house 14 feet 

 wide and 10 J feet high above the brickwork; 

 and such a structure might be used either for 

 forcing Grapes and Peaches, or as a plant-stove. 

 In some cases the bars for curvilinear houses 

 have been made of a uniform size; but although 

 these have been found to answer well for forty 



