65 
METHODS OF MEASURING SOIL TEMPERATURE. 
As it is very important that there should be numerous observations 
of soil temperature available for agricultural study, and as many 
persons are deterred by the expensiveness of the deep-earth thermom- 
eters, I would call attention to the fact that agriculture does not need 
to consider temperatures at depths below 4 feet. 
Several methods of measuring deep-earth temperatures have been 
most thoroughly studied in the memoirs of Wild and Leyst of St- 
Petersburg, a summary of which I have prepared and will submit 
at another time. For accuracy and convenience nothing can exceed 
the thermophone devised by Henry E. Warren and George C. Whipple, 
of the Massachusetts Institute of Technology. 
The soil thermometers constructed in accordance with suggestions 
made by Milton Whitney, of the South Carolina Experiment Station 
have been used by him at several stations and he has published a 
description of this new self-registering soil thermometer as follows 
(see Agr. Sci., Vol. I, p. 258; Vol. III, p. 261): 
This is a modification of Six’s form of thermometer in which the 
maximum and minimum temperatures are registered in one and the 
same instrument. The essential features of the thermometers are as 
follows: A cylindrical bulb 6 inches long, filled with alcohol. The 
bulb is protected by a somewhat larger cylindrical metal tube, con- 
taining numerous holes, and is to be placed 3 inches below the surface 
of the soil—i. e., so that the bulb will extend vertically between the 
depths 3 and 9 inches, respectively, in the soil. The tube carrying 
the alcohol extends some 6 or 8 inches above the surface of the ground, 
when it bends twice at right angles and descends again to the surface, 
bends at right angles twice, crossing the main stem, and is carried up 
about 6 or 8 inches again, where it terminates in a bulb partially filled 
with alcohol. The lower bend in this stem carries a column of mer- 
cury which is drawn back toward the bulb when the alcohol contracts, 
and pushes a steel index up to the minimum temperature on a scale 
which reads downward. This index is held supported in the alcohol 
by a lttle spring when the alcohol expands and the mercury leaves it, 
while another index is pushed up to the maximum temperature by the 
other end of the column of mercury. The indices are set by the help 
of a magnet. 
The advantages claimed for this instrument are that it gives at once, 
without any calculation, the mean temperature of a definite depth of 
soil, for which we now use at least three thermometers, while it gives in 
addition the maximum and minimum temperatures, and need only be 
read once a day instead of three times, as at present. * * * 
Thermometers can be made, of course, with bulbs longer or shorter 
than the one described. We adopted the length of 6 inches placed 3 
inches below the surface, as in our experience that represents a layer 
of soil in which most of the roots of the cotton plants are contained. 
We expect to distribute a number of these instruments through the 
