TREES AS GOOD CITIZENS 
164 
be repeated in two or three inch layers until the filling is 
complete, and the tamping should always be directed 
toward the back of the cavity rather than toward the 
bottom. The filling should stop at a point even with the 
surface of the wood. To allow the cement to project 
outside the wood, so that it is flush with the outer surface 
of the bark, will delay the purpose for which the operation 
was performed, as such projection will prevent new tissue 
from covering the opening without first lifting the edge of 
the old bark and growing up over the cement. If the filling 
is flush with the surface of the wood and the cambium has 
not been injured, a healing callous will promptly grow over 
the edges, and if the wound is small this new growth may 
eventually cover the entire surface. In overlapping the 
edges of the cement, this new tissue completely seals the 
cavity and usually keeps out moisture in small cavities. 
After the cement has dried, its surface should be coated 
with coal tar, to prevent unnecessary absorption of water. 
Often creosoted wood blocks, sawdust and tar are used 
instead of cement and have some advantages, especially 
less weight, without marked disadvantages. 
If the cavity is long and deep, leaving little more than 
a shell of a trunk, the tree should be removed unless there 
is some very special reason for trying to prolong its life. 
If it is determined to fill the cavity, the tree should be 
braced and guyed and then the cavity should be reinforced 
by means of one or more bolts. A cavity less than two 
feet long usually requires no such reinforcement, but in 
those of greater length a bolt every two feet is desirable. 
The bolting is done in fashion similar to the process of 
bolting split crotches. The bolt should be half an inch 
thick, or even larger for trunks of considerable size, and 
must fit snugly into the hole bored for its reception. 
It should be placed where the nature of the cavity indi- 
