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proper time for study and for rest : even then some- 

 times without success. This end would be much more 

 effectually and certainly secured by a complete system 

 of covering hot-houses and forcing-houses ; and this 

 plan would secure the further advantage of avoiding 

 the undue stimulation of the plants by a then unneces- 

 sary amount of heat, applied solely to prevent the very 

 evil which covering also prevents, namely, the risk of 

 excessive cold during the night. 



The principle upon which a covering acts most effi- 

 ciently, is that of enclosing a complete body or stratum 

 of air exterior to the glass, this body of air being 

 entirely shut away from the surrounding outer atmo- 

 sphere. Air being a bad conductor of heat, the 

 warmth of the interior is by this means prevented from 

 passing to the exterior atmosphere ; or, in other words, 

 the exterior atmosphere, being prevented from coming 

 in contact with the glass, cannot absorb from the in- 

 terior any material proportion of its heat. To secure 

 this advantage, however, the coverings must be kept 

 from contact with the glass, and they should extend on 

 every side where the structure is formed of materials 

 which readily conduct heat such as glass or iron. 

 The coverings should in fact form neither more nor 

 less than a close outer case. 



One point connected with the application of these 

 coverings, which I consider would constitute an im- 

 provement, and which, as far as I am aware, has never 

 been acted on, is that of having them to fit so accu- 

 rately as to exclude the external air (a matter of no 

 difficulty in the degree required), and then to have a 

 series of ventilators provided, to stand open during the 

 night, whereby an interchange of the atmospheric vo- 

 lume would take place throughout the night, without 

 exposing the plants to contact with cold air. The 



