and 



VOLUME OF THE STEM. 31 



S = ~= -780 X d- {= r X tt), 



V = -0796 X 'f X //, 



or 



V = -785 X cl' X H {= r X vr x H). 



In practical work, the sectional areas are taken from specially 

 prepared tables ; there are also tables which give directly the 

 volume of logs according to their mean girth and length, or their 

 mean diameter and length. (See Appendix I., page 335.) 



All these calculations are made on the assumption, that the 

 section represents a circle. This is, however, rarely the case. 

 As a rule, the degree of divergence from the circular shape 

 depends on : — 



(1.) The part of the stem; the lowest and uppermost parts 



differ most. 

 (2.) The age of the tree ; young trees are more regularly 



shaped than old ones. 

 (3.) The species. 



(4.) The conditions under which the tree has grown up ; in 



crowded woods the shape is more regular, than in the 



case of trees grown in the open ; exposure to strong 



winds, slanting position and the nature of the soil also 



affect the shape. 



Generally, the sections of trees approach the shape of an 



ellipse, the greater axis of which lies, in the same locality, as a 



rule in a constant direction. Where trees are much exposed 



to wind, the greater axis lies generally in the direction of the 



prevailing wind : in Western Europe, therefore, from west to 



east or from south-w^est to north-east. 



The inaccuracy caused by measuring the girth, and calcu- 

 lating therefrom the sectional area, has been found to amount, 

 on an average, to about 7 per cent. ; w4iere only one diameter 

 is measured, the error may be the same or even more ; where 

 two diameters at riLrht angles are measured aud the mean 



