72 ON VENTILATING HOTHOUSES, AND GLAZING WITH SHEET GLASS. 
creates the most pleasurable sensations, and speaks volumes of the triumphs of modern 
horticulture. The question then arises, in the course of this argument, can drip he pre- 
vented by any known means ? and, if so, can it be done without the sacrifice of any 
material point as to the welfare of the plants ? I feel assured it can ; and I will proceed 
with all deference to offer my reasons for so thinking. 
It has been urged as an argument against night coverings that they cause plants to 
“ draw,” as it is technically termed. Of course, if the night temperature of a Cucumber 
house is kept in the dead of winter (or indeed in summer even) at a much higher 
temperature in its relation to the day heat, and to the light, than is necessary, the plants 
will indeed become “ drawn,” or long-jointed ; especially if the air is devoid of motion, and 
carrying much atmospheric moisture. But is it fair to say, that this is chargeable on 
night covering ? I can readily believe that a Cucumber house glazed with the sheet glass, 
in large panes, and possessing a cover on the roof, would endure a temperature of 50° 
at night without any material damage ; but I should hesitate to face a temperature of 55° 
without the covering. Now if such can be proved, and I hesitate not to affirm that it can, 
the necessary consequence is, that night coverings are great economisers of fuel ; and that 
in their adoption, having by such a function once covered their first cost, all the fuel 
saved afterwards becomes clear gain, besides carrying out a great principle ; for vegetation 
in general has no affection for fire-heat ; it is merely a necessary evil. Thus much will, 
I think, tend to show that there is not of necessity 'positive harm, in night coverings ; 
I proceed now to show that they are antagonistic to the drip before complained of. 
It is well known that the drip in our houses is occasioned by the moisture-loaded air 
coming in contact with the roof, or cold outside walls ; in the main, however, it may be 
considered as almost entirely originating with the roof. Now, some degree of atmospheric 
moisture there must be, in all houses, even in the winter conservatory, inasmuch as if no 
special provision be made for the production of it, such will naturally emanate from the 
soil in which the plants are growing. A covered roof, therefore, will be found entirely to 
prevent condensation, provided it never be suffered to freeze ; and that some escape for the 
air-suspended vapour be provided in the back wall, at a high level ; and that such escape or 
ventilator be open all night : this I have long proved to my entire satisfaction. 
From the foregoing considerations, therefore, it appears tolerably plain, as I think, 
that, before long, it will be deemed expedient to fit up the roofs of forcing houses and 
conservatories, or structures for Orchids, with coverings *on rollers, which shall be equally 
adapted for warding off extreme sun-light, for softening the rigour of severe weather, and 
for preventing drip in our plant houses. 
The expense of such will of necessity be taken into consideration ; but if their utility 
be once admitted, I make no doubt that a rather general demand will soon produce a fabric 
adapted to the purpose. I have one in use in an Orchid house, which was obtained from 
Mr. Yexley, of Merton, who, it appears, has endeavoured to meet the growing wants of 
the gardening public in these things ; it cost some four or five pounds for an Orchid house 
forty feet in length, and is most easily worked on a roller. I have used it both as night 
covering and as shading ; and it appears to answer such purpose with reference to Orchids 
extremely well, the house being glazed with the sheet glass ; it is a lean-to structure. 
I. would now, in conclusion, beg to offer a few remarks on ventilation, and some other 
matters, about which both the theoretic and the merely practical horticulturist still differ. 
Follow Nature, observe both parties; but how can Nature be followed as long as an 
artificial atmosphere is allowed to be confined for a long night in our forcing houses? 
The admission of fresh air, night as well as day, is a point which, some seven years since, 
was scarcely mooted. By degrees, however, the old “coddling system,” happily termed so 
by Dr. Lindley, was broken into, and first-rate practitioners, as well as the amateur, now 
boldly call for air — fresh air — night as well as day. Is it not time, then, that the lovers 
of horticulture should make up their mind on this point ; and having made it up, to carry 
out such principles, by providing for a constant renovation of the atmosphere of our 
hothouses, night as well as day ? for Nature does not pause in this matter. 
With regard to ventilation, as bearing on the question of sheet glass, there can be no 
