By the Rev. Thomas Wilkinson. 163 



than when the rajs fall perpendicularly. But at the angle 

 of 34°, the two days when the rays are perpendicular being 

 near each other, a great part of these advantages is lost, 

 and on the contrary, the reflected rays are nearly the most 

 possible, which they would actually be if the angle were de- 

 pressed only 6° more. Besides, the good effects of the incli- 

 nation are the greatest when least requisite, July 21. The 

 diagrams will shew the angles of altitude and elevation of 

 the roof. 



In Fig. 1, HN is the horizon, OQ the equator, P the 

 pole, Z the zenith, S and s the sun, O the earth, then the 

 sun's altitude SON = QON ± SOQ = compt. of lati- 

 tude ± sun's declination. In Fig. 2, ABC is the section 

 of a Forcing-house, SGD a perpendicular ray, then as the 

 triangles BGD, ACB are similar to the angle, GDB 

 = angle CAB, i. e. the angle at A between the wall and 

 roof = angle of sun's altitude when his rays fall perpendi- 

 cularly. 



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 SO 



474 



80 



412 



77 30 



356 



75 



299 



70 



222 



65 



157 



