ADMISSION OF alt AND LIGH 319 
the West Indies chilly and cold nights usually succeed the 
hottest days, they will imitate nature, by shutting up the 
nouse 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 deducible 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, 
