By John Frederic Daniell, Esq. 23 



of radiation, to which the glass is particularly exposed, which 

 would reduce the saturation to 434°, and this is a degree of 

 drought which must be nearly destructive. It will be allowed 

 that the case which I have selected is by no means extreme, 

 and it is one which is liable to occur even in the summer 

 months. Now by an external covering of mats, &c. the 

 effects of radiation would be at once annihilated, and a thin 

 stratum of air would be kept in contact with the glass which 

 would become warmed, and consequently tend to prevent 

 the dissipation of the heat. But no means would of course 

 be so effective as double glass including a stratum of air. 

 Indeed such a precaution in winter seems almost essential to 

 any great degree of perfection in this branch of Horticulture. 

 When it is considered that a temperature at night of 20° is 

 no very unfrequent occurrence in this country, the saturation 

 of the air may upon such occasions fall to 120°, and such an 

 evil can only at present be guarded against by diminishing 

 the interior heat in proportion ; but whether we run upon 

 Scylla or Charybdis is no very desirable choice. 



By materially lowering the temperature we communicate 

 a check which is totally inconsistent with the welfare of tro- 

 pical vegetation. The chill which is instantaneously com- 

 municated to the glass by a fall of rain and snow, and the 

 consequent evaporation from its surface, must also precipitate 

 the internal vapour, and dry the included air to a very con- 

 siderable amount, and the effect should be closely watched 

 I do not conceive that the diminution of light which would 

 be occasioned by the double panes, would be sufficient to 

 occasion any serious objection to the plan. The difference 

 would not probably amount to as much as that between hot- 



