OBSERVATIONS ON THE ROOFING OF HOT-IIOUSES. 71 
the colder body constantly receiving the particles of heat which float 
(surrounded by a vesicle of water) in the surrounding medium. Such 
condensations, however, cannot with propriety be called dew, though it 
is very common to say, “ the marble is dewed.” A decanter of cold 
wine or water brought to table in warm weather, is said to be “ soon 
dewed; ” but the processes are dissimilar. In the last-mentioned 
instances of condensation, there is great disparity in the temperatures 
of the dewed body and the medium whence the moisture is drawn; 
whereas, in respect of those slender, unsubstantial leaves of grass which 
carry most dew, it cannot be supposed that their lower temperature is 
the attractive quality, because, if the experiments made by naturalists 
with delicate thermometers be true, the interior of free-growing plants 
is always of higher temperature than the air around them ; and in the 
case of the hotbed dews there both the vapour and the plants are equal 
as to temperature. It is true that the lowest stratum of the air in con¬ 
tact with the surface of the ground, is necessarily colder (in consequence 
of radiation and evaporation) than the superstrata; yet it does not 
follow that the leaves of healthy vegetation immersed in it are still 
colder, to attract and condense water as a block of marble does. 
Whatever may be the cause of the formation of dew, and whatever 
the source whence its globules are collected, I need not endeavour to 
explain by mere conjecture; the foregoing description of the pheno¬ 
menon, under the various circumstances in which it appears and dis¬ 
appears, will serve to assist the inquirer who has pleasure in the study 
of meteorology. J. W. 
Observations on the Roofing of Hot-houses. —The earliest, 
and by far the most common, forms of glass roofs for hot-houses, are 
lean-tos against a wall more or less elevated in front. Such roofs are 
generally laid at about an angle of 45 degrees from the horizon ; this 
pitch being considered most receptive of the rays of light, and equally 
safe from leakage of rain, or from the direct pelting of hail-storms. 
Nar row houses for forcing peaches, are usually covered with single 
lights, the whole length of the rafter : wider houses, as graperies, have 
two tier of lights, the upper tier sliding down over the other, with low 
glazgd fronts and ends. 
This form is the most simple, and, as already said, the most common. 
Many opinions are held as to the proper angle in which sloping roofs 
should be laid ; and some persons have been at the pains to calculate 
the angles of incidence of the sun’s rays, from his rising to his setting, 
for every day in the year upon a plane surface ; and all this to ascer¬ 
tain which inclined angle would intercept the greatest number of the 
rays. We know not that any great good has resulted from the conclu- 
