36 



DESIGN. 



the altitude of the sun in the latitude of London, the impinging 

 ray falling on the roof, c, at an angle of 45°. It will be seen 

 that the angle, contained between the back wall of the house 

 and the inclined plane of the roof, c, is just equal to the sun's 

 altitude, — the one forming an exact perpendicular to the other. 

 Allowing, then, for the difference of altitude betwixt the 

 latitudes of London and Philadelphia, for instance, we have a 

 difference of inclination of about 11°. Hence the roof of a hot- 

 house, to receive the same influence of the sun's rays at thai 

 period, would be at an angle of 34°. The difference will be 

 more closely perceived by the following cut. 



Fig. 2. 



a- 



In this cut we have given the altitude of the sun at Philadel- 

 phia, a, with the roof at right angles to it, on an angle of 34°. 

 At ^, we have given the altitude of the sun at London, with its 

 corresponding angle of elevation, 45°, and, according to the 

 principle here laid down, both of these roofs should be equally 

 influenced by the sun, notwithstanding the difference of his 

 altitude at the respective places. 



In a theoretical point of view these principles are correct, and 

 are certainly preferable to the usual mode of putting on roofs 



