2C0 THE COTTAGE GARDENER AND COUNTRY GENTLEMAN’S COMPANION, January 13, 1857. 
the chamber from gaseous exhalation. With this view, let 
me urge upon all those who may employ the Polmaise 
principle to be extremely particular in the manner in which 
the stove is built. Let the outside be parged; let it be con¬ 
structed of at least nine-inch brickwork; and if the iron 
plate can be cast in one, with a projecting rabbet on its under 
surface, and this rabbet dropping into a sand groove, it 
appears that all possibility of exhalation must be precluded. 
Indeed, if even this were insufficient, it is hardly probable 
that a sound principle of heating should be lost for want of 
some ingenious mechanical contrivance to prevent exhalation. 
However, I have thought it my especial duty to point out 
the weak points of Polmaise; and while I acknowledge that 
I think the form of stove I have employed has many advan¬ 
tages, and especially the retention of the heat, I by no 
means wish to prevent others of a more complex and ex¬ 
pensive, though probably of a more powerful character, 
being employed. All the merits of my stove, whatever 
these maybe, are due to Dr. Arnott; but I trust that the 
great principle of Polmaise, the conditions under which the 
stove is placed, will never be lost sight of; for on no other 
principle cau atmospheric heat be so naturally, so cheaply, 
or so uniformly diffused. 
During the last two months I have had repeated oppor¬ 
tunities of making thermometric experiments in the hot¬ 
house at Nutfield, and also other trials of a very interesting 
character, and leading to very important practical conclu¬ 
sions. The hot air, as it enters the hothouse from the 
chamber, passes beneath the bottom of the plunging bed, 
exactly at its point of entrance ; one of the slates on which 
the bed rests has been removed, leaving an area of about two 
feet, or half the hot-air opening; and by this means a ther¬ 
mometer can be suspended in the hot blast at its entrance. 
I have seen this indicate 174° Fahr., but it commonly ranges 
from 120° to 150°. When about the latter point, another 
thermometer suspended one foot above the former will in¬ 
dicate from 85° to 90°; while a third, on the same level with 
the second, but at one foot horizontal distance, will in¬ 
dicate only 65°; and other thermometers hanging in different 
parts of the house, at one end or both, or in the centre, 
indicate G3°; while even a thermometer on the floor, at 
the extreme end of the house, will indicate 61°. The 
only unequal portion extends about two feet around the hot¬ 
air opening, and in the ridge of the house, where we must 
always expect to find the temperature several degrees higher. 
The temperature of the plunging bed varies from 80° to 
92° Fahr. in different portions, which is accounted for by 
the fact, that much of the hot air escapes through the open 
ventilators before reaching the extreme end of the bed ; 
but for many purposes this variation is desirable, and all 
this may be regulated, according to circumstancesj by the 
side ventilators. The temperature of the plunging bed is 
peculiarly steady, arising from the material being a bad con¬ 
ductor of heat, slow to heat and slow to cool. The inference 
to be drawn from all the experiments I have made is, that 
there will be no occasion to provide any means of distributing 
the hot air ; it will equally diffuse itself. Another point I 
have determined is, that the lower the external temperature, 
the greater the proportionate difference between the hot¬ 
house and the external air, arising partly from the principle 
of compensation already noticed. Thus the average tem¬ 
perature of the hothouse above the external air, with 
moderate fires at night, is about 20°; but on the only oc¬ 
casion in which I have yet had an opportunity of testing it 
in frost, the house at night was 30° above the external air, 
and never during the night was the difference more than 26°. 
It was a quiet, clear, frosty night, the external air at 31° 
•Fahr.* I find the uniform range of the differential tlier- 
* Another experiment was made by closing four of the cold-air orifices, 
reducing the exit of the cold air to two one-foot openings, and I could not 
detect that the temperature of the house was always affected, while it was 
quite evident that the velocity of the currents in these two was greatly in¬ 
creased ; neither could I ascertain that the uniformity of the temperature 
was at all affected. The drains left open were those at the extreme end of 
the house. Had these been closed, and those near the chamber opened I 
think it probable the uniformity would suffer. I also closed all the hot-air 
ventilation, allowing only the escape of hot air at the opening close by the 
hot chamber; neither did this affect the general uniformity of the dis¬ 
tribution. These evidently lead to the conclusion, that a small amount 
of cold-air drain will prove sufficient; and also that any chamber or 
means of diffusing the hot air is entirely unnecessary, except where re- 
quired, as at Nutfield, for the bottom-heat. This experiment has not 
mometer, from six o’clock in the evening till eight in the ' 
morning, is from 62° to 57° or 58°. It must be remembered 
that this is legitimate atmospheric temperature. There are 
no means within the house of affecting the thermometer by : 
radiation, either from pipes, or from bricks heated by pipes, ! 
or by hot flues, and then attributing the thermometric point 
to atmospheric temperature, which is partly owing to 
radiation, a very common error; but it is the temperature j 
of the air taken by a thermometer suspended against the j 
upright side-sashes of the house. The house itself is a span- 
roof, containing about 4000 cubic feet of air; it is glazed 
with sheet glass, the ends are both glazed, and the upright 
side-sashes are three feet in height, standing upon stono > 
walls four feet high. It is situated in a kitchen-garden, partly 
walled, on a very considerable elevation, as willbe evident when : 
I state that, though only twenty miles from London, I can 
from my own grounds, on the same level, distinctly see the 
Downs in clear weather. I have purposely abstained from 
using any covering material, such as asphalt shutters, with 
a view of testing the heating powers. These facts entirely 
corroborate those obtained at Polmaise by Mr. Murray, 
which were published in The Gardeners' Chronicle of the 
present year. 
TO CORRESPONDENTS. 
Dew (W. Elliott). —Dew is neither more nor less than moisture in¬ 
visible whilst dissolved in the air, rendered visible by being condensed 
from it by coming in contact with a much colder body. Saussure, 
Prevost, and others, have thought that electricity had something to do 
with the phenomenon, but they are all explicable by the laws known to 
regulate the effects of caloric. If you require full information on the 
subject, read Dr. D. P. Thomson’s Introduction to Meteorology. 
Plants from Abroad ( Italicus ).—The names are wretchedly spelt. 
We can only make out those numbered as follows :—1. Is a pretty Pea 
flower ; will do in a greenhouse in summer, but is most impatient of 
cultivation. 2. A pretty Convolvulus-looking climber, not difficult to 
manage in a greenhouse in summer, but very bad to keep in a stove in 
winter. 3. A Salvia-looking flower of no great beauty. 6. An old 
classical plant from the Cape, with Pea flowers and Mimosa-like leaves j 
would live out in the south of Ireland with a slight protection. 8. A 
tall, herbaceous, greenhouse plant, with flower-spikes like those of 
Verbena venosa ; not worth much. 9. A beautiful-leaved, greenhouse 
shrub in England, but never flowers; the flowers like a Locust-tree. 
10. An old Indian shrub, very handsome there, but not a favourite tree. 
11. A stove climber ; but this kind of Convolvulus is only a pretty little 
Indian annual, and only pretty in a wild state. A botanist collector 
gathers all seeds he meets with, an amateur only such kinds as strike 
his fancy, therefore his seeds are more worth notice; but the practical 
man at home can alone determine the value of such seeds. All in this 
list have been tried repeatedly, and found, with the exception of Virgilia, 
not to be worth culture. Virgilia Capensis, three or four years old, 
can be bought for a few pence. 
Small Gas -heated Conservatory (E, C.). —You shall be answered 
fully next week. 
Removing an Araucaria (H. L .).—The best time for removing 
your Araucaria nine feet high would have been in October, but if taken 
up with a good ball of earth during mild weather, removed to a dry soil, 
firmly staked with three stakes, and well mulched, we should not object 
to removing it this month. It requires a sheltered situation. All ever¬ 
greens are best moved in October. If by thinning you mean pruning, 
April is the best season for Laurels, Hollies, and such shrubs. 
Wintering Pelargoniums and Geraniums (-).—The large 
greenhouse would be the best if you had a stove to exclude the frost, 
otherwise keep them in your windows. See what Mr. Fish has said 
about “ Window Gardening in Winter.” It applies to your case 
exactly. 
Succession of Peas (A Young Gardener). — Fairbeard’s Champion 
of England and Hair’s Mammoth sown at the same time, and simul¬ 
taneous sowings of them repeated as soon as the plants from the next 
preceding sowing are ready for sticking, will keep you supplied. There 
are no better Peas grown. If you persist in having none but tall 
varieties, sow . Thurston’s Reliance instead of Hair's Mammoth. You 
might grow Vines and Peaches in pots at the south-west end of your 
Vinery. 
Charcoal (An Old Subscriber). —Can any one of our readers state 
how wood should be burnt into charcoal ? Except on a very large scale 
it seems to us a waste of material. 
Lucerne (A Subscriber). —It is sown not planted. The seed should 
be inserted during March or early April in shallow drills. 
Fruit-trees (A Constant Subscriber). —The Due du Telliers is a 
good Nectarine, and one worth cultivating. You may plant out fruit- 
trees in the open ground which have been grown in pots, and they will 
always been uniform in its results ; sometimes when only two cold drains 
are open the temperature of the house is less by two degrees than when 
all are open, but I can never detect that it affects the uniformity of the 
distribution. It must also be understood that the temperatures named 
are not always the same ; but the most important and best ascertained 
point is the equal diffusion of the heat. 
