FROST 
take only one-fourth as many heat units 
to raise the temperature one degree with- 
in such space. If one oil pot is provided 
for such a space, that is, one pot per 
tree, we will have 70 pots per acre. Hach 
pot will have to take care of 600 pounds 
of air. Most of the crude oils used as 
fuels for orchard heating in this dis- 
trict average nearly eight pounds per 
gallon, and it has been found by labora- 
tory test that a pound (one pint) has a 
calorific, or heat value, of about 18,000 
British Thermal units. Some oils test 
higher, some lower. In burning tests in 
the field under actual frost conditions it 
has been found that for the lard pail 
type of pot, such as the Bolton, with 
or without perforations in the upper 
rim, two pounds of oil are consumed per 
hour. Naturally, the oil consumption is 
greater when the pots are first lighted, 
and this is also true where there is con- 
siderable air movement. Of course, com- 
bustion is not perfect, hence the total 
calorific power of the oil is not utilized. 
However, since we are dealing only in 
round numbers we will suppose that 
combustion is fairly complete. Then two 
pounds of oil will give off 36,000 heat 
units per hour, or 600 per minute. Now, 
since the cubical space occupied by one 
pear tree contains about 600 pounds of air 
at our average pressure and ata tempera- 
ture of 32 degrees, it means that each 
minute 600 heat units are expended on 
600 pounds of air, or sufficient to raise 
the temperature of this mass of air 
through four degrees Fahrenheit. We 
have not taken into account the small 
amount of water vapor present under 
frost conditions, as this would not ap- 
preciably affect the calculation. It is 
supposed, of course, that the air is not 
in motion, and that there is no radiation 
of heat beyond the imaginary cubical 
Space occupied by the tree. In actual 
practice we know that radiation does 
take place, and that there is usually 
some air movement. Of course, this is 
offset to a very great extent in old or- 
chards by the trapping of the heat and 
the braking effect on wind currents, due 
to the extended branches, but in young 
1027 
orchards, covering but a small ground 
area, air movement and radiation are 
practically the same as in the open. 
There is one thing to be said, however, 
under our conditions. Upward radiation 
of heat is not so great as one would sup- 
pose. During the past four years a large 
number of observations have shown that 
the temperature of the atmosphere dur- 
ing a freeze rarely reaches the danger 
point at a height of 15 to 20 feet above 
the level’ of the valley floor. Since this 
is true, there would be no tendency for 
heat to be radiated from below into this 
upper stratum of warmer air—in fact the 
heat movement would rather be the re- 
verse, that is, downward. As previously 
explained in another part of this arti- 
cle, frosts which occur in this valley are 
due to depression rather than elevation. 
It is the cold air coming from very high 
elevations in the surrounding mountains 
that flows downward into the valley floor, 
tending to push the warmer air upward. 
For a while radiation from the ground, 
which has taken in heat during the 
hours of sunshine, tends to warm this 
cold air. But to return. We have shown 
that with no wind and with one oil pot 
for every pear tree the temperature may 
be raised four degrees per minute within 
the calculated space. But if the air 
moved only 100 feet per minute, or a 
little more than one mile per hour, the 
temperature could never rise more than 
one degree above the temperature of the 
incoming cold air. At four miles per 
hour it could rise but one-fourth degree. 
This would be true only in the outside 
tree rows, on the side from which the 
air movement comes. For all the rows 
beyond the outside row, some of the 
heat units generated in the first row 
would be added to the heat generated in- 
side. This interesting calculation shows 
that an orchard in the form of a solid 
Square would not be so difficult to save 
from frost injury as one of the same area 
of only a few rows. During the past four 
seasons this has been demonstrated in 
several of our orchards. In the Potter 
and Goold orchard the main body of 
pears has easily been saved when tem- 
