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 tt, 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, ee however, was of opinion that 
1 
