56 TROWNBRIDGE: THE THERMONETRIC MOVEMENTS 
5. The degree of moisture in the atmosphere, as shown by 
‘relative humidity observations, has apparently a negligible effect 
on the position of the branch. 
6. Modulus of elasticity experiments seem to show that there 
is a stiffening in the branch below freezing (32° F.); but a stiffening E 
of the branch should produce a less mobility or less depression of 
the branch whatever the stress; hence the slight effect observed 
due to a change in the bending constant of the branch arising 
from a reduction of temperature, taken alone, is in the opposite 
direction to the gross changes of the level of the branch observed 
at low temperatures. 
7. Observations on frost cracks of the linden have shown that : 
the width of the longitudinal cracks follow the changes in air tem- 
perature approximately below 32° F. When the crack first opens 
it is not thermometric because it is greatly affected by strain 
conditions, and resistance in the tearing of the wood. 
Thermometers placed within the tree trunk at different dis- — 
tances from the surface indicated that the cracks were not caused 
by an unequal strain between the central core of the tree and the 
portion of the tree near the surface, arising directly from a differ- _ 
ence of temperature, but due rather to excessive tension conditions 
in the living portion of the tree directly below the surface of 
the trunk. 
All observations made indicate in a general way that the 
bending of the branches is closely related to the process which 
produced the longitudinal frost cracks in the linden. 
8. Preliminary measurements indicate that the coefficient of 
expansion in the living linden in the transverse directions of the 
wood directly below the bark is very large below freezing, while 
small in the longitudinal direction. 
9. Experiments on a Paulownia (Paulownia tomentosa) and a 
sycamore tree (Platanus orientalis), show scarcely a trace of the 
thermometric movement as exhibited by the linden. 
The writer begs to acknowledge the courtesies shown by 
Professor Harper and other members of the Department of Botany 
at Columbia University, who have made many helpful suggestions 
pertaining to the botanical side of the present investigation. 
PHOENIX PHYSICAL LABORATORY, 
COLUMBIA UNIVERSITY 
