1026 
Pig. 3. 
degree of the correct temperature read- 
ing, since he is quite certain to keep 
on the safe side at all times. Besides 
the thermometers in the field, the frost 
alarm thermometer, which is designed to 
awaken one when a certain temperature 
has been reached, may be more or less 
advisable. Of course, all a frost alarm 
thermometer can do is to indicate that 
a certain temperature has been reached. 
It is usually made to ring at, say 33 or 
32 degrees, and does not necessarily in- 
dicate that dangerous temperatures will 
follow. During the past the local fore- 
casting station has indicated very nearly 
the hour when it would be necessary to 
fire as well as forecasting the possible 
temperature, so that with this in mind 
those who had no frost alarm thermom- 
eters got along very well with a good 
alarm clock. It would be a serious mis- 
take to begin lighting up without know- 
ing whether or not the temperature 
would go below the danger point; this 
is where the local forecaster’s work is of 
greatest value. 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
ON! 
% : 
3 
3 
3 
g 
..4 
g 
: 
Carbondale Frost Alarm. 
Physics of Orchard Heating 
In all that has been published no 
figures have been given to show what 
a certain quantity of fuel will do under 
actual conditions. Of course, this can- 
not be accurately stated, but we can 
give what we might expect under set 
or ideal conditions. We will take as an 
instance the protection of a pear 
orchard with the trees set 25 feet apart 
on the square. With the trees in good 
bearing the maximum height at which 
fruit is borne is not more than 15 feet, 
and is usually much below this. We will 
consider each pear tree as growing in a 
cubical space which, under normal at- 
mospheric pressure at our. elevation 
above sea level (1,400 feet) and at a 
temperature of 32 degrees Fahrenheit, 
contains, in round numbers, 600 pounds 
of air. If this space contained 600 
pounds of water it would require 600 
British Thermal heat units to raise the 
temperature through one degree Fah- 
renheit, but since the space is filled with 
air under the above conditions it will 
