OF ATMOSPHERICAL TEMPERATURE. 143 



are when vegetation is most active. On the other hand 

 countries nearer the equator are subject to seasons 

 of dryness, the continuance of which is often much 

 greater than anything we know of here in the open 

 air, and consequently artificial means must also be 

 adopted to bring about, in glazed houses, that state 

 of things at particular periods; namely, those of the 

 repose of plants. These 1 facts afford abundant proof 

 of the necessity of regulating the moisture of the 

 atmosphere with due precision. 



By throwing water upon the pavement of glass 

 houses, by means of open tanks of water, by reser- 

 voirs placed upon them, by syringing, and by other 

 contrivances,* the quantity of water in the air may 

 be easily increased, even up to the state of saturation. 

 But there are some circumstances, easily overlooked, 

 which interfere very seriously with this power, and 

 which, it may be conceived, may reduce it very much 

 below the expectations of the cultivator. 



The most unsuspected of these is the destruction 

 of aqueous vapour, by the hot, dry, absorbent surface 

 of flues. The advantages derived from hot-water 

 pipes, or steam pipes, over brick flues, are so well 

 known, as not to require any evidence to prove the 

 fact. Gardeners explain the difference in the action 

 of the two, by saying that the dry heat produced by 

 hot- water pipes is sweeter than that given off by flues ; 

 which is not a very intelligible expression. The fact 



* A discharge of steam into a glazed house has occasionally been 

 employed; but the*-method requires much attention on the part of 

 the operator, and seems inferior to other contrivances. 



