338 FOECING. GARDEN. 



render the sashes immovablej and even to loosen the 

 walls ; and the contraction being apt to fracture the 

 glass,, and to produce openings between the sashes at 

 which hoar-frost may enter. 



In order to secure the greatest possible influx of light, 

 scientific horticulturists have proposed hot-houses with 

 curvilinear roofs. It was remarked.by Sir Geptge Stuart 

 Mackenzie, to whom the merit of the proposal is pri- 

 marily due, that if we could find a form for a glass-roof, 

 such that the sun's rays should4)e perpendicuUr to some 

 2')art of it, not on two days, but during the whole year, 

 that form would be the, best. Such a figure is the sphere, 

 and he therefore proposes a quarter segment of a globe, 

 or semidome, the radius of which is iibout fifteen feet. 

 The frame for the glass-Ajork is formed of equal ribs of 

 hammered iron, fastened into an iron plate in the para- 

 pet wall, and fixed at top into an iron ring connected 

 with the back wall. There , are no rafters or sliding 

 sashes, J)ut air is admitted by ventilators in the parapet 

 and back walls-. .". 



This form of hot-house roofs was warmly patronized 

 by the late Mr. Knight, who, however, was of opinion 

 ^that the house proposed by Sir George Mackenzie was 

 too high, in proportion to its length and brejidth, and 

 therefore recommended a smaller section of a sphere, 

 with a greater radius. His dimensions are forty feet 

 long, fourteen wide in the centre, and, including the 

 front parapet, twelve feet fiigh. The late Mt Loudon, 

 who, it is. believed, was the firfft' that actually erected 

 hot-houses on this principle, proposed s.everal sub- 

 varieties of form. He describes [Enci/c. of G-ard.) the 

 acuminated semidome, the acuminated semiglohe, the 

 shniellipse, and the jjarallelogram ivith'curved roof and 



