ADMISSION OF AIR AND LIGHT, 319 



the West Indies chilly and cold nights usually succeed the 

 hottest days, they will imitate nature, by shutting up the 

 house by day, and throwing it open at night, This prac- 

 tice, however, supported as it is by analogy, is subject to 

 many limitations, and can only be followed in our climate 

 during the summer and autumn months. It is useful, not- 

 withstanding, to remember the principle, though it admits 

 only of partial application. 



The admission of Light. In addition to the heat with 

 which natural light is always accompanied, there seems to 

 be another property necessary to vegetation, which from 

 some cause hitherto unexplained, is partly deranged by its 

 transmission through glass. The fact, however, is evident, 

 from the circumstance that plants thrive better near glass 

 than at a distance from it, though the intensity of light is 

 apparently undiminished. Hence practical gardeners are 

 anxious to distribute their finer plants in situations as close 

 as possible to the glazed roofs of hot-houses. 



Connected with the admission of light is the determina- 

 tion of the pitch or angle of elevation of the roofs of glazed 

 houses. It is evidently of advantage that the rays of light 

 should fall upon glass perpendicularly, as loss by reflection 

 is then a minimum, or indeed little or nothing. The angle 

 necessary to obtain this result is easily dedueible from the 

 sun's place in the ecliptic. At the equinoxes, the sun's 

 meridional height above the horizon at any point of the' 

 earth's surface is equal to the complement of the latitude 

 at that place ; and hence, in order that the sun's rays may 

 be perpendicular at that period, it is only necessary to make 

 the elevation of the roof of the hot-house equal to the lati- 

 tude of the place. The angle for any other season may be 

 obtained by subtracting from the latitude the declina- 

 tion of the sun, if at that time to the north of the equator, 



