82 STRUCTURES AD.4PTED TO PARTICULAR PURPOSES. 



As the influence of the upper and lower strata of air, in large 

 houses, will be discussed in a subsequent portion of this work, 

 devoted to that subject, we will not enlarge further upon it at 

 present, more than to observe, that lofty-domed, or curvilinear 

 roofs, as that lately erected at Kew, are more difficult to manage, 

 both in winter and summer, than low-roofed houses, whether 

 curved or straight, and that the impossibility of rendering these 

 houses in any way workable, has induced, in some instances, 

 their almost entire abandonment on the part of the proprietors, 

 owing solely to the intense heat of the superior regions of the 

 house. 



The most experienced and enlightened men have satisfied 

 themselves, that structures in which the atmosphere has to be 

 kept at a higher temperature than the external atmosphere, and 

 in which plants have to be grown, should be kept at the very 

 lowest elevation which the use and purpose will admit, so that 

 the temperature of the air, at the level of the floor, and among 

 the roots and lower portions of the plants, may be as little dif- 

 ferent as possible from what it is in the higher regions of the 

 house; by regarding which, the house will be much easier kept 

 during summer, with respect to air and moisture, and, during 

 winter, with respect to a more equal diffusion of heat. 



In the comparatively still atmosphere of a hot-house, when all 

 is closely shut up in a cold winter's night, the difference betwixt 

 the temperature of the atmosphere at the surface of the floor 

 and the highest part of the roof will generally be in the ratio 

 of one degree to every two feet of elevation ; thus, in a house 

 20 feet high there will be a difference of 10°, and in a house 60 

 feet high the same rule gives a difference of no less than 30 de- 

 grees. This ratio, however, is not absolutely correct, as we 

 have proved by experiment, in houses of various sizes, which 

 give, under certain circumstances, a greater diflference of tem- 

 perature than here stated, as will be shown when we come to 

 treat on this branch of horticultural science.^ 



We have already said enough on this point, here, to show the 

 advantage of erecting low-roofed conservatories, especially when 



* See Ventilation. 



