38 



DESIGN. 



from various angles between the plane of the horizon, and within 

 two and a half degrees of the perpendicular. 



Bouguer's Table of Rays reflected from Glass. 



Of 1000 incidental rays, when the angle of incidence is 



87 30', 584 are reflected. 



85, 543 " 



82 30', ..... 474 



80, 412 



77 30', 356 



75, 299 



70, 222 



65, ...... 175 



The slope of hot-house roofs, therefore, should depend on the 

 following circumstances : 



The latitude under which they are erected. If in a southern 

 latitude, the plane of the roof should be as oblique as possible 

 to the sun's rays. South of 40, the angle of incidence should 

 not be less than 20. It will be recollected that this angle is 

 contained between the sun's rays and a perpendicular to the 

 roof. 



The position of the house, and the purposes for which it is 

 intended. Houses intended for the forcing of fruits in winter, 

 may have their roofs made on a perpendicular to the sun's rays 

 at that season. Conservatories attached to dwelling-houses may 

 also have their roofs perpendicular to the rays of the winter sun, 

 for the same purpose ; but blinds should be provided for them, 

 during the months of summer, to guard against the effects of 

 the perpendicular rays when the sun is crossing his meridian 

 altitude, 



