1826.] Mr. Daniell on Climate. YXI 



is absolutely necessary that the process of transpiration should 

 proceed ; but there is no danger that the high temperature of 

 the hot-house should ever attain the point of saturation by 

 spontaneous evaporation. The temperature of the external air 

 will always keep down the force of the vapour ; for as in the 

 natural atmosphere the dew-point at the surface of the earth is 

 regulated by the cold of the upper regions, so in a house the 

 point of deposition is governed by the temperature of the glass 

 with which it is in contact. In a well ventilated hot-house, by 

 -watering the floor in summer, we may bring the dew-point 

 within four or five degrees of the temperature of the air, and the 

 glass will be perfectly free from moisture ; by closing the venti- 

 lators we shall probably raise the heat 10 or 15 degrees, but 

 the degree of saturation will remain nearly the same, and a 

 copious dew will quickly form upon the glass, and will shortly 

 run down in streams. A process of distillation is thus esta- 

 blished, which prevents the vapour from attaining the full elasti-? 

 city of the temperature. 



This action is beneficial within certain limits, and at particular 

 seasons of the year, but when the external air is very cold, or 

 radiation proceeds very rapidly, it may become excessive and 

 prejudicial. It is a well known fact, but one which I believe 

 has never yet been properly explained, that by attempting to 

 keep up in a hot-house the same degree of heat at night as 

 during the day, the plants become scorched ; from what has 

 been premised it will be evident that this is owing to the low 

 temperature of the glass, and the consequent lovv dew-point in 

 the house, which occasions a degree of dryness which quickly 

 exhausts the juices. 



Much of this evil might be prevented by such simple and 

 cheap means as an external covering of mats or canvass. 



The heat of the glass of a hot-house at night cannot exceed 

 the mean of the external and internal air, and taking these at 

 80° and 40°, 20 degrees of dryness are kept up in the interior, 

 or a degree of saturation not exceeding 528. To this in a clear 

 night we may add at least 6° for the effects 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 exter- 

 nal 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 con- 

 sequently 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 



