236 



PRINCIPLES OF GARDENING. [CH. VII. 



that direction. To ascertain how a glass roof may 

 be constructed so as to receive the greatest number 

 of rays of light from the sun pei-pendicularly or 

 near to perpendicularity at any given time of the 

 year, it is necessary to know the latitude of the 

 place where the hot-house is erected, and the sun's 

 declination at the period when most light is required. 

 The latter information may be obtained from most 

 almanacks, and if it be subtracted from the latitude, 

 the remainder will be the angle desired. 



If London be the place, and May the 6th the 

 time about when the most light is desired, the 

 latitude being .51° 31' and the smi's declension then 

 16° 36' north, therefore the roof ought to slope at an 

 angle of 34° 55^ 



