316 



SCIENCE OF GARDENING. 



Part II. 



ellipsis, or oval, and in the superstructure {b, b) one-fourth part of the solid figure. Its ad- 

 vantage over a semi-dome is, that, whether the trees are to be trained on a trellis parallel 

 to the class, or against the back wall, a greater surface for training is obtained in propor- 

 tion to the volume of cubic air to be heated- On the other hand, its glass surface is less 

 perfect in respect to perpendicularity to the sun's rays ; though in this respect the differ- 

 ence is not of great consequence. Houses of this sort, Adanson informs us, are of Dutch 

 invention. As the sun retired from them in the afternoon, the eastern parts of the ellipse, 

 as they fell successively into shade, were covered with reeds or mats ; and, in like manner, 

 in the morning the east end was uncovered first, and the west end only as the sun came 

 round on it. {Families des Plantes, vol. i. Pref.) 



1607. The parallelogram with curved roof and ends {Jig- 256.) is one of the most con- 

 venient forms of curvilinear roofs for the common purposes of culture, as it admits of 

 more regular figures of beds, paths, trellises, &c. within, and of every variety of dimen- 

 sion. In regard to light, heat, and beauty, they do not differ materially from the semi- 

 ellipse. Of this form, a considerable number of forcing-houses, and some green-houses, 

 have been erected. Among the latter may be noticed one for Messrs. Loddiges, and 

 another for the Horticultural Society. {See the Table, 1587.) 



256 



1608. The ridge and furrow roof may be effected either in curvilinear or right-lined hot- 

 houses ; and consists in placing the bars in the rebates of which the glass is put, in such a 

 manner as that the section of the roof may always be a zig-zag line, in which the space 

 traversed by each side or zig may either contain several bars {Jig. 257.), or merely one 

 pane of glass. {Jig. 258.) In both modes it is generally desirable, that the ends of all 



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the bars should terminate in one horizontal line on the top of the parapet ; which need 

 not, however, be the case in their termination against the back wall. Some apparent 

 difficulty of glazing is thus occasioned in the lower part of the roof; but the difficulty 

 is only apparent, for as smaller and only triangular pieces of glass can be used there, 

 it becomes, in fact, more economical, by occasioning the use of pieces of glass which 

 would otherwise be thrown away. The advantages of ridge and furrow roofing are 

 chiefly obtainable in countries liable to heavy falls of snow or hail, and in houses which 

 are parallelograms in plan. Almost any weight of snow may be carried by such roofs, 

 especially when the bars are not far apart, as the pressure will evidently be almost 

 entirely on the upper bars, and not on the glass. As to hail, as it will always meet the 

 surface of the glass in a ridge and furrow roof at an angle of 45°, it can never do it 

 much, if any, harm. Curvilinear houses with roofs of this description are therefore 

 peculiarly suitable for the north of Europe, and especially for Russia; and in houses 

 with triangular and straight-lined roofs, the sun {a, Jigs. 257, & 258.) will be perpendi- 

 cular to half the roof (by being so to half each ridge) at that period which forms half 



