of double-roofed Hot-houses. 



537 



the same fire ; or the back wall may be flued, and pipes in- 

 troduced both into the house and, between the front and side 

 walls, &c. &c. 



Air may be admitted either through ventilators in the 

 walls or in the sashes ; and, by forming ventilators alternately 

 in the upper and lower sashes, the coldness of the external 

 air will, in some measure, be taken off, by passing through 

 the warmed stratum contained between the lights. In spring, 

 when a more abundant supply is necessary, the doors may be 

 partially opened, or some of the inner sashes may be entirely 

 removed, and the outer ones opened. 



The expense of double glazing, and the consequent loss of 

 light, may be urged as objections to these structures ; but 

 when the immense labour of covering every evening with 

 straw mats and shutters, the breakage of glass, and the enor- 

 mous consumption of fuel, are taken into consideration, the 

 advantage will in a short time be found to be on the side of 

 double sashes. In respect to the light, when the rafters are 

 neatly made, very little will be lost, and the deprivation, at a 

 season when the plants are not in a growing state, is far less 

 injurious than the scorching heat, which, when the house is 

 formed only of single lights, is requisite to resist an intensity 

 of cold sufficient to lower the thermometer 20°, or even 25°, 

 below zero of Reaumur. 



Since the above-described house was erected, there has 

 been a double-roofed camellia house built for the Archduke 

 John, near the Carolinen Thor, at Vienna, of which Jig. 85« 

 is the ground plan. 



85 



ft. 6 



) I I 1 'I I 

 4 2 



HE 

 18 



30 ft. 



a, Stage. 



b b, Furnaces for heating the vacuity; 



