18 



THE GREEN-HOUSE. 



down and open, or draw up and shut, at pleasure ; 

 and the doors being generally at the ends, the keep- 

 ing them open answers for these parts of the struc- 

 ture. In houses with fixed roofs, and more especially 

 in curvilinear houses, it is common to have opening 

 sashes in front, and opaque shutters in the top of 

 the back wall which open, and by means of these 

 a current is made to ascend through the house. In 

 some houses, as in that of Lord St. Vincent already 

 referred to (p. 15), ventilation is effected solely by 

 shutters in the front wall below the glass, and others 

 'in the back wall close under the angle of the roof. 



It remains only to consider, how far the form of a 

 green-house is influenced by considerations relatively 

 to the weather, and especially to rain, snow, hail, and 

 frost. As to rain, a certain degree of slope in the 

 roof is necessary to throw it off when accojnpanied by 

 wind, and experience has pointed out 40 and 45 de- 

 grees of inclination as the two extremes. The maxi- 

 mum of that inclination is most favourable also for 

 throwing off hail when it falls perpendicularly ; and 

 preventing snow from accumulating in large masses, 

 so as to break the glass by its weight. No slope of 

 roof, however, will guard against hail accompanied 

 by wind ; all that can be done, where there is much 

 to be dreaded, is to adopt panes of glass of a small 

 size and good quality — say not broader than seven 

 inches, and of Newcastle rather than Greenock manu- 

 facture. To guard against the breakage of glass by 

 frost, the slope of the roof should not be less than 45 



