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JOHN H. EHLERS 
sents a temperature difference between these junctions of 0.04° C, 
i. e., enough heat is lost by conduction from junction 2 to reduce its 
temperature 0.04° C. below the true temperature of the oil. When it 
is borne in mind that the temperature gradient between air and oil in 
this experiment was 9.7° C, and that the maximum temperature 
gradient between leaf and air under natural conditions was 8.83° C. 
and the average considerably below this, it is evident that the error 
arising from loss of heat by conduction along the lead wires is also neg- 
ligible. Furthermore it is compensated for in part by the very small 
increase in temperature due to wounding. 
(5) Absorption of Radiation hy the Junction in the Leaf. — It may 
have occurred to the reader that the rise in temperature indicated by 
a junction placed in a leaf exposed to direct sunlight may be due in 
part to the absorption of radiation by the junction itself. Smith 
(28) has shown that this is not true. By alternately shading a junction 
by means of a small piece of pith and exposing it to direct sunlight, 
he found that direct sunlight had no effect upon the junction whatever. 
He concludes from his experiment that "the rise of temperature of a 
junction registered when it is placed in a leaf exposed to direct sun- 
light must all be due to the absorption of radiation by the leaf, and 
no part of it to the absorption of radiation by the junction itself." 
Smith's result was accepted by the author and no experiments were 
made to verify it. 
12. Meteorological Data, (i) Air Temperature. — In the earlier 
experiments the air temperature was obtained with a mercury thermo- 
meter graduated to degrees Centigrade. Later a registered thermo- 
meter graduated to .1° C. was used. No attempt was made to read 
the air temperature beyond one tenth degree. The thermometers 
were hung behind the screen upon the platform. 
(2) Solar Radiation. — For purposes of comparison, the intensity 
of the solar radiation was also taken. For this a black bulb in vacuum 
thermometer graduated to the Fahrenheit scale was employed. It 
was placed on the platform near the leaf. For the sake of uniformity 
the readings were converted into degrees Centigrade. 
(3) Velocity of the Wind. — For this a self-registering anemometer 
making contact every one tenth mile was used. The miles of wind 
per five minute periods could be determined. The anemometer was 
placed upon the platform near the leaf; the registering device in the 
laboratory. 
