14 
EOrAL HORTICULTURAL SOCIETY. 
them more pervious to fluids, and therefore no longer capable of 
retaining their cell contents. 
It appears ' to me that the pyramid fruit-trees and espaliers, 
which are now so much grown, are peculiarly exposed to the effects 
of frost, as they are pruned so that each branch overhangs, and 
consequently protects any below it to the least possible extent. 
It ought, nevertheless, to be possible to devise some cheap and 
effective way of protecting temporarily trees of this small size from 
frost. In view of any expedients of this kind, any means of 
anticipating their need would be of the greatest value. It may be 
well, therefore, to mention that in spring a dry state of the air, 
indicated by any very considerable difference in the readings of the 
dry and wet bulb thermometers, is likely to be followed by frost. 
The reason is simple : the night frosts, which injure vegetation, 
arise in the main from the loss of heat from the earth's surface by 
radiation. If there is much moisture present in the air this loss 
of heat is impeded. The luminous heat radiated from the sun 
passes through atmospheric moisture with little impediment, but 
the obscure or non-luminous form in which the earth radiates it 
back again is caught by it, as it were, in a trap. On May 1 7, at 
Elackheath, near London, the air was nearly saturated with 
moisture, the degree of humidity being represented by 94 deg., 
and the lowest temperature of the air by 44 deg. Both tempera- 
ture and humidity fell, pari passu, till May 20, when the first 
stood at 32*6 deg., and the other at 69 deg. It would be of the 
more importance to have warnings of the probable occurrence of 
low temperature, because Mr. Glaisher has shown from the 
Chiswick observations that periods of deficiency of temperature 
below the mean are often prolonged to as much as a fortnight. 
In the forty-four years there were eighty such. I feel strong hopes 
that the telegraphic communications about the weather, which the 
Meteorological Office now collects from stations in the British Isles 
and Western Europe, will eventually lead to warnings of probable 
falls of temperature being obtained. 
The apparently paradoxical fact that the temperature often 
falls lower, and plants correspondingly suffer more from frost, in 
low grounds than in those which are adjacent and higher, has 
often been observed, and it is important to remember it as a 
practical point in planting and laying-out grounds.* The ex- 
* See Dines " On the Temperature of Hill and Valley," in the Journ. 
