330 VENTILATION WITH FANS. 



in relation to any kind of horticultural structures, then a supply 

 of warmed air must be kept up by means of the heating appa- 

 ratus, and a channel of conduction for the vitiated air to 

 escape by. 



In places where the mechanical power for moving- a fan can 

 be easily obtained, this machine may be turned to excellent 

 advantage. The question, therefore, is not as to the adaptability 

 of the machine, but as to the means of working it so as to bring 

 it within the reach of hot-house adaptation, at a cost which 

 would justify us in recommending it. 



There are various points to be considered in relation to draw- 

 ing in fresh, and'expelling foul, air from a hot-house, namely, that 

 we must not only expel the vitiated air from the house, but we 

 must introduce pure air into its place ; and that pure air must 

 be warmed before it is introduced. We have heard and read a 

 good deal about this and the other method of introducing warm 

 air into a hot-house ; and, in theory, many of these notions are 

 very plausible, but when we come to apply them to practice, 

 they are entire failures. 



The principal objects to be obtained by an efficient system of 

 night ventilation may be classed as follows : — 



1. The expulsion of a certain quantity of vitiated air, in a 

 certain time, from the whole volume contained in the house ; 

 and, as the impure air rises by rarefaction to the upper regions 

 of the house, means must be provided to carry it away, with- 

 out creating counter-currents, or admitting any cold air, by the 

 channels of conduction thus made. 



2. A quantity of air must be introduced to the internal vol- 

 ume equal to the quantity expelled ; otherwise the remaining 

 internal volume will expand, by its increased temperature, and 

 fill the space occupied by the decreasing volume, and thus the 

 air becomes more vitiated than if none had escaped. The air 

 thus broucfht in must be introduced without acting- in a direct 

 current upon the vegetable productions within the house. 



3. The air thus introduced must be warmed to a certain tem- 

 perature, before it enters the house. This temperature should 

 be regulated by the temperature at which it is desired to main- 

 tain the internal atmosphere. If the desired temperature be 



