u 



mode of measurement recommended gives an exciss of __.^5n?."^*. that is to say, 



an excess equal to the area of a circle whose diameter is equal to half the difference of 

 the two measured diameters. Save in very exceptional cases this difference is small 

 enough to he negligible. The area of sections of irregular contour can be determined 

 from the mean of three diameters ; but the result thus obtained will generally be 

 found to he somewhat too hij^h. 



Diameters are measured with calipers such as the in- 

 strument already described at page 47. This instrument, it 

 will have been noticed, resembles in all its essential parts a 

 shoemaker's measure. In measuring logs and trees the 

 following general rules should be borne in mind : — 



(i) Diameters, in experiments and in accurate valuations, 



are to be preferred to girths, 

 (ii) In the case of stems of elliptical or oval shape take 



the mean of the largest and smallest diameters, 

 (iii) In the case of all large stems measure at least two 



diameters, 

 (iv) In the case of stems of irregular shape measure 

 several diameters and avoid all protuberances, etc. 

 (v) Measure diameters and girths always in a plane at 



right angles to the axis of the stem, 

 (vi) If the place of measurement falls on an irregular 

 part of the stem, measure the diameter or girth, 

 as the case may be, at equal distances above 

 and below the irregularity and take the mean of 

 the two measurements, 

 (vii) Moss, etc., thick enough to vitiate the measure- 

 ment of the stem, should be removed, 

 (viii) If a very accurate measurement of an irregular 

 section is required, transfer its outline to tracing 

 paper and compute its area with a planimeter or 

 acre-comb, 

 (ix) Pelled trees should be divided in the usual way, 

 that is to say into logs and smaller pieces, and 

 should then be measured, 

 (x) Never be without tables showing at a glance the 

 areas of circles for given diameters and girths. 

 Several formulae, approaching more or less to accuracy, 

 have been devised for the determination of the contents of 

 round timber; but only two are of practical utility. They 

 are — 



(1) volume of contents of log = half the sum of the 



areas of the top and bottom sections x the 

 length ; 



(2) contents = area of mid-section x length. 



