grt] BRIGGS & SHANTZ—SOIL MOISTURE 215 
shaft. The propeller is driven by a horizontal belt from a small 
vertical shaft motor mounted on a bracket at the top of the barrel. 
The lower end of the propeller tube extends nearly to the bottom of 
the barrel, while the upper end fits into the center of the solid shelf 
on which the pots rest. An annular space is provided between the 
shelf and the barrel, The operation of the stirrer causes a circu- 
lation downward through the central tube, upward along the walls 
of the barrel, and radially inward across the shelf and around the 
pots. While the temperature of even this large volume of water 
will vary somewhat during the day, these changes are very gradual, 
and are found to produce no detrimental effect. For the purpose, 
a simple arrangement of this kind is preferable to a more elaborate 
thermostat. 
_ Assoon as the plants in a pot show unmistakable signs of wilting, 
the water content of the soil is determined. The pot is inverted, 
and the soil mass removed intact by gently jarring the edge of the 
_ pot against the bench in the usual way. The lower two-thirds 
of the soil mass is taken for the moisture determination, since the 
Toots do not usually develop so extensively in the upper portion. 
The moisture determination is based upon the loss of water taking 
place when the soil is dried to constant weight at 100° C., the per- 
centage of moisture being based upon the dry weight of the soil. 
Composition of the wax seal 
For sealing the pots, we have tried paraffin, petrolatum, beeswax, 
and tallow in various proportions. Of these, we have found a 
wax composed of 80 per cent paraffin (melting point 45° C.) and 
20 per cent petrolatum to be most satisfactory for use at ordinary 
temperatures, exact proportions not being important. T his mix- 
ture melts at so low a temperature and has such low heat conduc- 
tivity that it can be poured into a pot around the most delicate 
Seedling without injury. This is an important point in the case of 
delicate dicotyledons. On cooling, this wax adheres well to the 
glass and to the soil, forming a perfect seal. Other mixtures, 
notably those containing beeswax, show a decided tendency to 
‘eparate from the glass on cooling, necessitating resealing the edges 
with a hot iron. 
