320 FORCING GARDEN. 



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 

 general elevation of 34 for the latitude of London, an 

 angle which corresponds to the 20th of May and 21st of 

 July. This would afford four months,. from the 20th of April 

 to the 21st of August,, during which the angle of incidence 

 at mid-day would not at any time amount to 9 r while the 

 deviation at the winter solstice would be 48, and the loss 

 of light from reflexion would be little more than 1 -30. The 

 Rev. Mr. Wilkinson recommends 45^ a pitch extremely 

 suitable for early vineries and pine-stoves. In this case,. 

 the midsummer deviation would be 19,, and the loss 1-40, 

 and the midwinter deviation 30, while the loss is nearly 

 the same. From these statements, however, and from an 

 inspection of the table already referred to, it is manifest 

 that much greater exactness has been sought in this matter 

 than is at all necessary. The reduction of the opacity of 

 the roof, arising from the breadth and depth of rafters and 

 astragals, is of much greater consequence. Accordingly, 

 in some glazed houses, particularly those constructed of 

 metallic substances, rafters have been omitted altogether ; 



* The following is part of Bouguer's Table of Reflexions. Of 1000 inci- 

 dent rays, when the 



Angle of incidents is 75 299 rays are reflected. 



70 222 



65 157 



60 112 



50 57 



40 34 



30 27 



20 25 



10 25 



' 1 23 



