1028 
peratures ran as low as 20 degrees out- 
side, while two rows of pear trees ex- 
tending beyond the main body of the 
pear orchard but surrounded by apple 
trees fully larger lost most of their crop, 
although protected by a greater number 
of fires. No orchard heating device on 
the market effects perfect combustion of 
erude oil or distillate, therefore the theo- 
retical figures given above are hardly 
approached in practice. In some tests 
carried out in this district the beneficial 
results of certain devices did not show up 
when it came time to harvest the fruit. 
The time to tell whether orchard heating 
has been successful or not is when the 
fruit is picked and brought to the pack- 
ing house. Just to make fruit stick upon 
the trees is not protection. A misshapen 
or frost marked fruit is not commercial 
either for fancy box trade or for the can- 
nery. Canneries do not want badly frost 
marked pears, as the waste is too great. 
In the above calculation we have consid- 
ered only crude oil, but practically the 
same figures will apply to all the heavier 
distillates. It might be well to mention 
something in regard to other fuels we 
have used in this district. A pound of 
dry pine wood, under perfect combustion, 
will generate about 6,000 heat units. A 
pound of oak contains practically the 
same number of heat units. Coal, under 
the same conditions, has approximately 
12,000 heat units. The average weight of 
a cord of pine is about 2,000 pounds, and 
that of oak is about 4,000 pounds. These 
figures are, of course, only approximate, 
but they will serve as a basis for calcu- 
lation in case anyone should desire to use 
wood or coal for orchard heating pur- 
poses. The use of wood and coal has 
been discussed in previous articles, also 
in United States Farmer’s Bulletin No. 
401, which may be obtained by address- 
ing a letter to the United States Depart- 
ment of Agriculture, Washington, D. C. 
Meteorological Instruments 
Wherever it is found necessary to pro- 
tect orchards from frost injury each fruit 
grower should provide himself early in 
advance of the season for firing not only 
with fuel, pots or other heating appara- 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
tus, but also with a sufficient number of 
thermometers. It is also advised that 
each fruit grower should have a good 
maximum-minimum theremometer. <A 
dew-point apparatus or psychrometer for 
determining the dew-point temperature, 
accompanied with tables, would also be 
a valuable part of the equipment. The 
dew-point apparatus is simply two fairly 
good thermometers fixed together, with 
one of the bulbs covered with linen. A 
string tied into the rings of sufficient 
length to whirl the instrument completes 
it. In using the instrument, wet the 
covered bulb and whirl rapidly so that 
evaporation will take place from the wet 
surface. When the mercury in the wet 
bulb thermometer cannot be lowered any 
Fig. 4. Thermograph, or Self-Registering 
Thermometer. 
further it should be read simultaneously 
with the dry bulb theremometer. The 
readings are referred to tables (See U. S. 
Farmer’s Bulletin No. 401) which give 
the dew-point temperature. The dew- 
point temperature, when found in the 
early evening, is usually in close agree- 
ment with the minimum temperature the 
following morning, providing the sky re- 
mains clear and there is no wind. This 
is true during only a part of the year. 
The following data taken from the rec- 
ords made by the Medford United States 
Weather Bureau station for the years 
1909, 1910 and 1911 will show that the 
above statement holds good. It will be 
noted that the dew-point temperatures 
observed, both when frosts occurred and 
when they did not, agree fairly well with 
the minimum temperature: 
