ADMISSION OF LIGHT. 321 



but this kind of structure leads to considerable difficulties 

 in the admission of air. 



We have taken it for granted that the framework is com- 

 posed of wood ; and if prime Baltic timber be procured, 

 it will endure for nearly half a century. But in some 

 cases rafters and sashes made entirely of metal, generally 

 either malleable or cast iron, have been employed ; and in 

 others, a middle course has been steered by adopting wood- 

 en mortices and metallic tenons. The great objection to 

 the use of metal for rafters and sashes is, that it is too 

 rapid a conductor of caloric, and too liable to contraction 

 and expansion from the alternations of heat and cold ; the 

 expansion tending to render the sashes immovable, 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 George Stuart 

 Mackenzie, to whom the merit of the proposal is primarily 

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

 the sun's rays should be perpendicular to some part 

 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 semi- 

 dome, the radius of which is about fifteen feet. The frame 

 for the glass-work is formed of equal ribs of hammered 

 iron, fastened into an iron plate in the parapet wall, and 

 fixed at top into an iron ring connected with the back wall. 

 There are no rafters or sliding sashes, but 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 

 14* 



