THE GREEN-HOUSE. 



19 



degrees, and the panes should not overlap one ano- 

 ther more than a quarter of an inch. By attention 

 to these two points, no water will ever lodge in the 

 interstices between the panes, and consequently, there 

 being none to freeze, there will be no panes broken. 

 It is frequently alleged that the frost breaks glass by 

 contracting it, or by contracting the bars in which it 

 is glazed ; but this is an erroneous conclusion, made 

 without adequate observation. Nothing is more 

 clearly ascertained, than that the power of frost to 

 break glass, is nothing without the presence of water 

 in the interstices : this water, in the process of freezing, 

 expands and separates the two plates of glass, which, 

 being unelastic, necessarily break ; the fracture gene- 

 rally commencing at the middle of the lower part of 

 the pane where the water, from its gravity, had ac- 

 cumulated in the greatest quantity. 



It may be observed, with respect to the angle of 

 forty-five degrees recommended as best for the roof 

 in regard to weather, that it is also considered best 

 in all common forms, in respect to the sun's rays. 

 It was that adopted and recommended by Miller, 

 Speechly, Abercrombie, Nicol, and most of our emi- 

 nent practical gardeners, and as the Rev. W. Wilkin- 

 son has shown, with great reason, as it admits the 

 sun's rays to pass perpendicularly through the roof 

 during the two seasons of the year when they are 

 most wanted ; viz. in April for perfecting blossoms, 

 and in autumn for maturing fruits, and ripening 

 young shoots. 



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