336 FORCING GARDEN. " 



their finer plants in situations as close as possible to 

 the glazed roofs of hot-houses. 



Connected with the admission, of light is the deter- 

 mination of the pitch or angle of elevation of the roofs 

 of glazed houses. It is evidently of advantage that the 

 rays of light should fall upon glass perpendicularly, as 

 loss by reflection is then a minimum, or indeed little or 

 nothing. The angle necesssary to obt-ain this result is 

 easily deducible from the sun's place in the ecliptic. 

 At the equinoxes, the sun's meridional height above the 

 horizon at any point of the earth's surface is equal to 

 the complement of the latitude of that place; and hence, 

 in order that the sun's rays may be perpendicular at 

 that period, it is only necessary to make the elevation- 

 of the roof of the hot-house equal to the latitude of the 

 place. The angle for any other season may be ob- 

 tained by subtracting from the latitude 'the declina- 

 tion of the sun, if at that time to the north of the 

 equator, or by adding it if to the south.* . These periods 

 are of course selected in accordance with the time at 

 which the direct rays are most required. Mr. Knight 

 proposes a gdneral elevation of 34° for the latitude of 



^ The^ following is part of Bouguer's Table of Reflexions. 

 Of 1000 incident rays, when the . 



Angle of incidence ig 75° 



