14 



THE GREEN-HOUSE. 



of green-houses transparent, so it has been found ad- 

 vantageous to have this transparent tegument of a 

 particular form or inclination of surface relatively to 

 the rays of the sun. It is found that when the sun 

 shines obliquely on any body, whether transparent or 

 opaque, a great many of his rays pass off ; and con- 

 sequently, in the case of opaque bodies, do not com- 

 municate their heat to them, and in the case of trans- 

 parent ones do not penetrate through them. It is 

 therefore desirable to have the glass roof of green- 

 houses as much as possible in a form or slope which 

 shall form right angles with the sun's rays. But as 

 the sun changes his position in the heavens every day 

 in the year, and every hour in the day, how can this 

 be done ? The answer to this (and which was first 

 given by Sir George Mackenzie, a learned horticul- 

 turist) is, ' Make the surface of your green-house 

 roof parallel to the vaulted surface of the heavens, or 

 to the plane of the sun's orbit :' — in other words, let 

 the roof of your structure be a dome of glass, with 

 only a small part (about one-third of the whole) on 

 the north side opaque. On such a roof the sun's 

 rays will fall in a perpendicular direction in one point, 

 whenever he shines, throughout the whole year. A 

 most elegant house on this principle (fig. 6.) was 

 erected for the late Lord St. Vincent, by Messrs. W. 

 and D. Bailey, of Holborn, London. 



