98 DETERMINATION OF THE INCREMENT. 



tion the species, quality of locality, and the adopted method oi 

 treatment. Such woods are produced if no extraordinary in- 

 fluences have interfered with their progress, such as natural 

 phenomena, faulty treatment, etc. 



3. Local and General Yield Tables. 



If a yield table has been prepared for a particular district of 

 limited extent, it is called a " local " yield table ; if for a whole 

 province or county, a " general " yield table. 



The question, what limits should be assigned to the applic- 

 ability of a yield table, is still under discussion, but so much 

 is certain, that in the preparation of such tables a considerable 

 extent, of country can be thrown together without incurring any 

 appreciable inaccuracy. 



4. Quality Classes. 



Localities of different quality or yield capacity produce 

 woods, which follow in their development different laws. The 

 law of increment of the one cannot be evolved out of that of 

 the other. The preparation of a yield table should therefore 

 be based on data obtained from localities of precisely the same 

 quality. Practically, however, an immense number of different 

 qualities exist, hence in practical forestry some concession 

 must be made, by being satisfied with a limited number, which 

 rarely exceeds five, and frequently three are quite sufficient. 

 The best quality is generally designated as I. quality (though 

 the reverse would be better). 



In proceeding to construct yield tables it is obviously of the 

 first importance to have a ready method by which the quality 

 of a locality may be indicated. It has been explained in 

 " Sylviculture " that the several factors of the locality, such as 

 the chemical and physical conditions of the soil and subsoil, 

 the climate, etc., do not enable the forester to determine the 

 quality of the locality for forest purposes with any degree of 



