Part IV.] 
PRUNING AND THINNING. 
251 
girthing perhaps seventeen feet; not an incon¬ 
siderable plant certainly, but small for so great 
an irrigation. Adanson’s guess was made by 
cutting into the stem of the tree till the width 
of three hundred rings was measured. “ The 
average rate of growth of younger trees of the 
same species was then ascertained, and the cal¬ 
culation made according to a supposed mean 
rate of increase.” I quote from the admirable 
Lyell, who quotes the “ Biblioth. Univ.” on the 
longevity of trees. 
If the general average width of the rings, which 
included the growth of the young trees, was only 
the twenty-ninth part of an inch, what was the 
average of the first three hundred rings, which 
were all old growths ? Perhaps the fiftieth part 
of an inch. But Adanson should have given us 
these data, and his mode of calculation from them. 
1 cannot help thinking that he may have made a 
slight mistake in these calculations; that he may 
have omitted to perceive that, although to make 
up the diameter of a tree from the rings on one 
side of its centre the width of these rings must be 
reckoned double, to ascertain the age of the tree 
the number of the rings on one side of its centre 
must be reckoned single. The width of a half 
