VENTILATION AND COVERING OF HOT-HOUSES. 



29 



it has been found that a depressed rather than an elevated tem- 

 perature is desirable for the healthy development of all plants 

 requiring artificial heat, inasmuch as they will not thrive with- 

 out their natural season of repose. 



From the changeable nature of our climate, there is some dif- 

 ficulty in apportioning the degree of applied heat so as to suit 

 exactly the requirements of the plants ; and it is especially diffi- 

 cult to maintain with certainty the low degree of night tempera- 

 ture which would be desirable, and at the same time avoid risking 

 the safety of the plants through a sudden and unexpected declen- 

 sion of the temperature of the exterior air. These difficulties 

 under present circumstances have to be surmounted by a degree 

 of watchfulness and care which presses heavily upon the daily 

 rest of those whose duty is thus involved, and incapacitates them 

 to a certain extent for those studies which are indispenably con- 

 nected with honourable proficiency in their vocation ; nor is this 

 watchfulness, though rigorously maintained, at all times attended 

 with success. 



It appears to me that the end in view would be much more 

 effectually and certainly secured by a complete system of covering 

 hot-houses and forcing-houses ; and this, too. would very greatly 

 relieve the attendants. I am aware that night-covering is a 

 generally acknowledged benefit, and that it is in some cases acted 

 on ; but it should be more universally and more systematically 

 followed up. To assist in bringing the principle of night-covering 

 into more universal application is the purpose of the following 

 suggestion. 



I need not stop to show how night-coverings prove beneficial. 

 It is sufficient to state that whatever prevents the radiation of 

 heat from the interior to the exterior atmosphere through the 

 conducting agency of the glass, decreases in the same ratio the 

 amount required of applied heat, and hence saves the plants from 

 being submitted to unnecessary excitement. The principle upon 

 which a covering acts most efficiently is that of enclosing a com- 

 plete body or stratum of air exterior to the glass, this body of 

 air being entirely shut away from the surrounding outer atmos- 

 phere ; and as air is 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 interior any sensible proportion of its heat. To secure 

 this advantage, however, the coverings must be kept from con- 

 tact 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, in fact, should form neither 

 more nor less than a close outer case. 



