VENTILA TION OF GLASS-HOUSES. 



77 



is easy in theory but difficult in practice, as plants and all organic beings 

 when at rest can very ill bear the air in motion. 



Whenever any portion of the air of a glass-house is cooled, it 

 becomes heavy, and falls ; whenever it is heated, it becomes light, and 

 rises. In a cold night the pipes are at ioo° Fahr., the glass down to 

 lO° : consequently the air whicli touches the pipes expands, becomes 

 lighter, and rises rapidly ; the air which touches the glass cools, 

 becomes denser, and falls to the bottom of the house. In any large 

 greenhouse the descent of the chilled air in a cold night is apparent 

 to the senses. 



From the weight of the cold air it is advisable that, as a general 

 principle, a considerable part of the heat should be applied as low as 

 possible. In a long house with excess of heat at one end the hot 

 air rises and travels along the roof ; whilst the cold air falls and 

 returns by the floor to the source of heat, where it again ascends. 



In making arrangements for the heating of any specified house, 

 the engineer should well consider all cooling surfaces, for he may be 

 sure that the cold air from all such situations will fall to the floors 

 as certainly as bullets would, if dropped from a similar position. 



In my Cucumber-house (fig. 92) and Melon pit, I let air into the 

 house in contact with the hot metal and warm water. This in effect 

 puts a pressure upon the air of the house, 

 and the vitiated air escapes through every 

 little crevice in the glass : this is a very 

 safe system in cold weather. 



In all ventilating and heating arrange- 

 ments, it is essential that the air should 

 preserve its moisture, and my evaporators 

 are useful to determine this point. When fig. 92.— Cucumber House, 



we warm the air it becomes too drying, and we must add water • 

 and we do this at my garden by using open heated tanks of water, 

 and by employing iron troughs arranged at the top of the pipes. 



On a cold night, the air when heated by the pipes rises to the 

 glass. It deposits aqueous vapour on the glass, and becomes drier. 



