PHENOMENA OF WINTER. 
251 
level form. With the abatement of duty, and the freedom from breakage, this 
sheet-glass becomes an article of real economy. It is also much more free from 
blemishes than was the old crown ; and thereby permits the passage of the rays 
more freely, and with less irregularity of refraction. 
Where a moist atmosphere is to be kept up, it is a question of some doubt 
whether a powerful, but diffused light is not preferable to a direct ray ; for by 
accumulation of drops of water myriads of lenses are formed, which act focally, 
and sometimes must be productive of injury ; as is the case almost beyond doubt 
when a crop of grapes, just in the process of stoning, become scalded. And as 
fruit may be, and is injured, flowers, doubtless, are equally exposed to accident. 
Expense must be always an important item in horticultural erections ; other- 
wise, sheet-glass, ground on one side to take off the polish, would be an admirable 
medium for the equable diffusion of a full glare of sun. Sliding screens of oiled 
muslin form a capital substitute, either by day or night. They should be made to 
run under fillets, so as never to be moved by wind ; and with this extra defence 
much fire by night would be spared, because the heat within being retained, the 
action of the flues or watercourses would be much reduced. By night, we require 
protection, not active heat : hence more is done, and with far less injury, by 
effectual coverings, than by fire. 
Nevertheless, as artificial heat is required, a furnace should always be employed, 
which will consume everything that is in any degree combustible. 
We have before alluded to this subject, and become more and more convinced 
of the extravagant waste that attends coke fires ; and consequently of the erroneous 
construction of the cylinder-boilers which are fed with such fuel at the top. 
A furnace, with double doors, bars, and other appurtenances, which would 
admit of a large mass of cinders, small, smouldering coal, (that termed the Moira, 
from Leicestershire, is the best) is to be preferred, whether water be employed or 
not ; and forcing will never be rendered safe or economical till such a hot-water 
furnace be constructed, and brought into complete and certain action. There are 
so many forms of hot-water boilers offered by advertisement, that it is difficult to 
decide upon their several merits ; however, there is one material point which ought 
to be considered of great moment, and that is, the avoidance or combustion of smoke. 
Coke and charcoal yield no black smoke, but both are too expensive ; small 
cinders, and lumps of Moira coal, yield it in diminished quantity ; but it is above 
all desirable to cause the distillation and combustion of the inflammable coal-gas. 
This can be promoted in two ways — the first is, to bring the body of fire most 
remote from the door, and nearest the flue-neck, to a state of bright red heat ; and 
then to feed the fire by additions laid in body 'upon a heated iron plate cast in 
front of the bars, upon which the coal becomes hot, parts with its hydro-carbon, 
which is therefore carried over the red-hot coal, and thereby is ignited. During this 
process of distillation, the coal in front is partially coked ; and being pushed 
forward when it becomes needful to feed the fire, it inflames immediately on 
