Mr. Williams, in reply, said he had had a piece of this canvas
stretched on a frame exposed all the winter, and the only result was to
make it rather dirty. He stretched it as tightly as he could in putting
it on, but when it got wet it became still more tight, and gave a
little again on becoming dry. It bore the weight of the snow which had
fallen very well, and two or three spadefuls had been added to try it.
He had a note from Mr. Prim, saying that at the Houses of Parliament
the screens last about two sessions, being washed once a week, and the
destruction is due to the wringing. But there is really no occasion for
this, for if you syringe the stuff well from the inside, you make it
sufficiently clear to allow the air and light to pass through, and it
would probably last many years. He had tried the experiment of dipping
it in a very weak solution of tar, but this had the effect of matting
together the fine filaments, so that it did not act so effectually as
a strainer. It acted best when wet, because the fine particles of soot
adhered to it, and moist weather was just the time when the greatest
quantity of soot fell. It might be easily tried in London squares to
aid in the growth of flowers; he found that the cabbage plants which
were so protected throve remarkably well, and he had no doubt that
if flowers were planted and a screen put over them until they were
ready to bloom, it would be a great advantage. The action of a little
peat on the top of a wall to protect fruit trees is very simple, and
the explanation was afforded by the experiments of Dr. Wells on dew.
The frosts which did the greatest mischief, were due to radiation
from the ground on clear nights; and it would be found that if one
thermometer were placed in a garden under an umbrella, and another on
the open ground near it, the differences of temperature would be very
considerable; on cloudy nights there was very little difference. Last
night there was only a difference of 2°, but a few nights before it was
6°. The period of greatest cold might not probably be more than hour,
but it would be sufficient to do a great deal of mischief, and anything
which would check the radiation would have the required effect. In the
case of loosening the fruit trees from the wall there was, probably, a
double action; it prevented the tree being forced on by the warmth or
the wall in the daytime, and also avoided the chilling effect at night,
a rough wall being a good radiator, and sinking to a low temperature.
He did not think there was much danger to be apprehended from wind,
because the canvas being so open, the wind would pass freely through
it; but he had not seen it subjected to any violent gale.