44 CHRISTISON—OBSERVATIONS ON 
form or with very rough bark, an error of the twentieth or even 
the tenth of an inch may be caused by the slightest shifting of 
the tape. But such trees should be rejected, at least for fine or 
frequent observation, and if we select young cylindrical stems 
with smooth bark, or even when it is rough, provided the rough- 
ness be regular and free from excrescences, it is possible, by 
adopting careful methods, and with-practice, to attain a wonder- 
ful degree of accuracy. I have frequently checked an observa- 
tion on such trees by repeating it three times, using a fine 
millimetre tape, and found a variation of not so much as half a 
millimetre. The method adopted by me is fully explained 
under the next head. 
GENERAL EXPLANATIONS. 
1. METHOD OF TAKING OBSERVATIONS.—The measured 
point, generally five feet above ground, is marked in white paint 
by several short horizontal lines round the stem. A short per- 
pendicular line at one of them that occupies the most prominent 
position marks the spot where the measurement begins. 
Chesterman’s steel tapes are used, one, graduated to twentieths 
of an inch, for ordinary observations, and another, of more slender 
make, graduated to millimetres, for finer work. In both, the 
ordinary ring is replaced by a square, slightly wider than the 
tape, and included in the graduation. 
In small stems the tape is held in position at the fixed 
starting point with the nail of the forefinger of the left hand, and 
the tape is passed round the stem with the right hand, and 
brought fairly over the square, which can be accurately done, as 
the square is wider than the tape. The amount is then read off 
at the outer edge of the square. For larger stems the process is 
the same, except that, to allow the observer to go round the tree, 
the square is kept in place by a “ brog,” which must be removed, 
the square being kept in position with the nail of the forefinger, 
in order that the measurement may be read off accurately. 
2. THE GIRTH OF A TREE usually signifies its circumference 
at five feet from the ground, or, in a short stem, at its narrowest 
point. But five feet was the height aimed at for observation 
whenever it was practicable. 
