224 PRACTICE OF VENTILATION. 



consume the juices, choke the respiratory organs, and speedily 

 destroy the object they attack. 



How to ventilate houses -without produomg results even more inju- 

 rious than the absence of free air, is a horticultural problem by no 

 means solved. It is necessary — 1, that the aii should be as warm aa 

 that of the house ; it is necessary — 2, that the gardener should have 

 the means of rendering the ventilation dry or damp at pleasure ; it is 

 necessary — 3, that the method should be simple and economical. It 

 was this which led to what is called Polmaise heating, which excited so 

 much discussion in the pages of the Gardeners' Chronicle some years 

 since ; and there is little doubt that when it was skUftdly applied Pol- 

 maise answered the three preceding conditions. But mechanical and 

 other difficulties having led to the abandoning that method, it seems 

 to be desirable to point out some of the other plans which have been 

 found to succeed more or less completely. 



In the invaluable collection of scientific papers, by Mr. Andrew 

 Knight, coUeoted into a volume, after his death, we find, p. 224, an 

 account of a curvilinear Vinery, in which he attributes the inferior 

 quality of ftueen Pines grown in it to ',' the want of efficient ventila- 

 tion;" and he proceeds to state how he remedied the evil by an 

 improved mode of ventilation. In this house he had acquired the 

 power of almost wholly preventing any change of air whatever ; and he 

 exercised that power too extensively, after the fruit was shown, and 

 particularly after a part of it had nearly acquired maturity. This led 

 him to adopt a mode of ventilation, from which he expected to derive 

 aU the advantages of change of air, without materially lowering the 

 temperature of the house ; and the success of it greatly exceeded his 

 expectations. He first formed certain cylindrical passages of nearly 

 two inches diameter through the front waU.. Through these, which 

 were placed eighteen inches distant from each other, along the whole 

 front wall of the house, the air, whenever the weathe» was warm, was 

 sufiered to enter freely, and its entrance at other times was more or 

 less obstructed in proportion to its coldness ; but it was never wholly 

 excluded, except dxiring the nights in very severe weather. The 

 passages through the front waU were placed at just such a distance 

 from the ground as would occasion them to direct the air, which 

 entered, either into contact with, or to pass closely over, the heated 

 covers of the flue. It consequently became heated, and was impelled 

 amongst the Pine-apple plants, which stood in rows behind each other, 

 each row of plants being so far elevated above that before as to keep 

 every plant at nearly an equal distance from the glass roof. A ther- 

 mometer was so placed as to be equally distant from each end of the 

 house, and he observed that the temperature of that part of the house 

 in which the thermometer stood was raised between 2° and 3°, when 



