RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 93 
ever, are not given here because the results do not coordinate closely 
with this set. 
3. It is noteworthy that the coefficients for all species in the lime- 
stone soil are relatively low, while the really high values are given 
by the heaviest clay. The latter fact, like the result in a strongly 
humous soil, is believed to be due to nonconductivity of the clay. 
4. In this case the correlation between wilting coefficients and 
moisture equivalent is a great deal better than the correlation with 
capillary moisture. In view of what has been said regarding the 
level of moisture maintained in each pan, it seems pointed to suggest 
that the wilting coefficient may depend in some measure on the degree 
of moisture to which the seedlings have become accustomed. It is 
only logical to suppose that, if abundant moisture tends to stimulate 
growth, the seedling may, when drought occurs, be relatively defi- 
cient in the carbohydrates which assist in osmosis. 
The moisture equivalent is a term devised by Briggs and McLane 
(113) to define the amount of water held by a soil against a definite 
external force. In the original experiments of these authors the force 
employed was a centrifugal force exerting a pull 3,000 times as great 
as the force of gravity. The small samples of soil were placed in 
finely perforated cans, which in turn were placed against the inside 
wall of a heavy cylinder. The latter was caused to rotate rapidly 
by direct connection with a motor. 
In this early work the writers seem to have made no attempt to 
correlate the moisture equivalents with wilting coefficients. There 
was, however, a fairly successful formula devised by which the 
holding power of the soil was related to the constitution thereof, as 
shown by mechanical analyses. This, it is believed, has been found 
of little use. 
It remained for Briggs and Shantz (114) to carry on the wilting 
tests which showed the real value of the moisture equivalent deter- 
minations. In these later tests the centrifugal machine was con- 
siderably improved and its speed automatically controlled, while 
being cut down to give a pull of 1,000-gravity, since it was found 
that the higher tension extracted relatively little additional water. 
As the result of some hundreds of wilting tests and comparisons witli 
the moisture equivalents of the same soils, it was found that from 
light sands to the heavier clays a linear relation exists between these 
two measures, which is expressed by the formula: 
„ T . U . nn . moisture equivalent 
Wilting coefficients 1M ^ m T - 
