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 that 

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

 more closely perceived by the following cut. 



Fig. 2. 



Jn 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 b, 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 



