1921] KORSTIAN—DIAMETER GROWTH 455 
as a rigid support for the instrument. As this belt comes in tan- 
gential contact with the tree at only a few points, it interferes very 
little with its normal growth. The recording apparatus consists 
of a drum carrying a record sheet, the drum being rotated by clock 
work. In order to minimize the effect of the shrinkage and expan- 
sion of the bark, it is 
rasped or shaved down 
so that a very thin layer 
of cork covers the living 
inner bark on small spots 
just large enough to give 
suitable bearings under 
the contact screw and 
under the end of the 
bearing lever. Changes 
in the diameter of the 
tree trunk, that is, be- 
tween the contact screw 
on the opposite side of 
the tree and the arm of - 
the bearing lever, are 
accurately traced on the 
sheet of record paper on 
the drum by the record- 
ing pen. In this way a: 
continuous curve is ; : 
traced on the revolvin g Fic. 1.—MacDougal dendrograph in operation 
a blue spruce, showing encircling belt of wooden 
drum as it passes beneath blocks and yoke of invar, but with tin cover removed 
the point of the pen, the _ to show recording mechanism. 
position of which is grad- : 
ually changed as the tree trunk expands in the process of growing. 
The instrument is protected from the weather by a tin cover 
attached to the tree. The clock requires winding and the record 
sheet must be changed at least every seventh day. When the 
instrument is once properly adjusted it requires no further adjust- 
ment except such as may become necessary owing to the growth of 
the tree. Fig. 1 shows the dendrograph on a blue spruce tree, with 
tin cover removed to show the recording mechanism. 
Bye. 
