32 MEASUREMENT OF FELLED TREES. 

 01' 



' 4 " 



and 



V='Q79QxcfxH, 

 or 



V='7S5xd*xH. 



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 App. D. pp. 390 to 393.) 



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) Part of the stem ; the lowest and uppermost parts 



differ most. 



(2) Age of tree ; } r oung trees are more regularly shaped than 



old trees. 



(3) Species. 



(4) 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 great axis of which lies, in the same locality, as a 

 rule in a constant direction. Where trees are much exposed 

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

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

 east or from south-west to north-east. 



The inaccuracy caused by measuring the girth and calcu- 

 lating therefrom the sectional area has been found to amount 

 to about 7 per cent, on an average ; where only one diameter is 

 measured the error may be the same or even more ; where two 

 diameters at right angles are measured and the mean taken, the 

 error generally does not exceed 2 per cent, of the true value. 



