72 GARDEN FARMING 



TABLE V. PER CENT OF RAYS OF LIGHT REFLECTED FROM GLASS ROOFS AT 

 VARIOUS ANGLES OF DIVERGENCE FROM THE PERPENDICULAR. (BOUGUER) 



EXAMPLE. If a ray of light strikes a pane of glass at an angle of 20, ap- 

 proximately 2.5 per cent of the efficiency of the light is lost by reflection. 



TABLE VI. ANGLES OF ROOF FOR DIFFERENT HEIGHTS AND WIDTHS OF 



HOUSES. (TAFT) 



This table gives the angles which the roof makes with the horizontal. The 

 height is the distance from a point directly under the ridge to a point on a 

 level with the bottom of the sash or eaves of the roof ; the width is the dis- 

 tance along this level to the eaves. 



Ventilation. Ventilation in greenhouses should not be looked 

 upon as a means of lowering the temperature, but as a means of 

 changing the air ; that is, pure air is admitted for the benefit of 

 the plants and for the purpose of increasing the growth, rather 

 than for the purpose of securing better temperature conditions. 



