Forest Mensuration 59 



Finally, the differences in volume shown by average trees of similar 

 description (i. e., differing but slightly in diameter, length, etc.) are 

 formed and rounded off in a manner causing the volumes to show a more 

 steady mathematic progression. 



2. Graphic method. 



The volume of each tree measured is entered as the abscissa on a 

 diagram-system of co-ordinates, whilst the diameters of the trees (or the 

 age, etc.) are registered on the ordinate axis. Similaritf of length is in- 

 dicated by color of mark representing the tree; similarity of locality is 

 indicated by the form of the mark (square, triangle, cross, circle, etc.). 



Corresponding marks are then joined by chains (having square, cir- 

 cular, triangular links) of the proper color. 



Finally, average curves as well as maximum and minimum curves 

 are drawn for the various colors and forms of marks. 



Maximum and minimum curves should not represent the very best 

 and the very worst possibilities; they should represent the average of 

 very good -and very bad trees. 



The graphic method is more reliable, because less depending on mere 

 figures, than the mathematical method. Both methods are frequently 

 combined. 



A number of complete tree analyses furnishes more reliable results than 

 a large number of mere volume measurements because it yields more 

 reliable curves (guide-curves) of development for one and the same lo- 

 cality, and because it prevents the forester from drawing curves of growth 

 at random. 



If the sample trees (or sample logs) are sawn up at a saw mill where 

 the lumber is properly graded according to the inspection rules prevailing 

 for the species in question, the volume tables may also give the actual 

 average output of specified trees in lumber of the various grades. 



SECTION II. INCREMENT OF A WOOD. 



PARAGRAPH XCI. 



INCREMENT OF FORESTS. 



The volume increment of the virgin forest is on the whole nill. 



In America the value increment of a primeval forest is based more on 

 a price increment of stumpage than on a volume increment of trees. The 

 volume increment, in addition, can scarcely be ascertained with sufficient 

 accuracy for a given piece of forest at a reasonable expense. 



In second growth forests, on the other hand, say in Virginia, an abso- 

 lute knowledge of the productiveness of the forest renders forestal invest- 

 ments safer in the eyes of the owner ; and the safety of the investment it is 

 which alone can tempt the capitalist to invest in forestry. A knowledge of 



