On the Form of Hot-houses. 



follows, that within the above limits, the sun's rays are per- 

 pendicular twice in the year, once in going to, and once in 

 returning from, the tropic. Hence then, having determined 

 in what season we wish to have the most powerful effects 

 from the sun, we may construct our houses accordingly by 

 the following rule. Make the angle contained between the 

 back wall of the house and its roof, = to the complement of 

 latitude of the place, + the sun's declination for that day on 

 which we wish his rays to fall perpendicularly. From the 

 vernal to the autumnal equinox, the declination is to be 

 added, and the contrary. 



To apply these principles to the construction recom- 

 mended by Mr. Knight, we have the inclination of roof, = 

 34°. Therefore the sun's altitude, when his rays fall perpen- 

 dicularly, is 56°, which, taking the latitude of London to be 

 51° 29', will happen when his declination is 17° 3 J', about 

 May 20, and July 21. Now we want the genial warmth of 

 the sun most in spring, and therefore, for general purposes, 

 that construction would perhaps be the best which gives us the 

 greatest quantity of perpendicular rays then. If the incli- 

 nation were 45°, the sun's rays would be perpendicular 

 about April 6th, and September 4th. This construction has 

 the following limit : the height of the back wall must not 

 exceed the width of the house, -f- the height of the front 

 wall. And as the rays would vary very little from the per- 

 pendicular for several days before and after the 6th of April 

 and Sepetmber 4th, the loss of rays arising from reflexion, 

 would, as appears from the annexed table, be nearly a mini- 

 mum. Even at the winter solstice, the loss by the obliquity 

 of the angle of incidence would be only two in 1000 more 



