45 



where d = diameter at the beginning of the year or period, and 

 D = diameter at the end of the same. 



As regards trees, the different concentric woody layers compos- 

 ing which are not distinguishable, or, even if distinguishable, are 

 not the same in number for each year, the procedure just described 

 is not applicable. In their case the only plan to follow is to measure 

 trees of known ages and growing under identical conditions of 

 soil, locality and leaf-canopy, and deduce the required increment or 

 increments by comparing the various figures thus obtained. \Vhere 

 trees of different known ages are not obtainable, there is no alter- 

 native but to select one or more plots representing the average 

 characters of the entire forest in respect of soil, locality, composi- 

 tion and density, and containing between them individuals of all 

 the age-classes. These sample plots should be properly thinned 

 from time to time, so that none but trees growing without any un- 

 necessary check may come under observation. In order to secure 

 this object more effectually, no suppressed or overtopped trees should 

 be measured, nor even those that have their tops exposed to the 

 sky if their crowns have a restricted development owing to lateral 

 pressure from their neighbours. Subject to these exceptions, the 

 girths of all the trees should be taken at regular intervals of one 

 to five years, according to the rapidity of growth of the trees. 

 Unless the trees increase in girth very rapidly, the interval should 

 not be less than two years, for not only is the increment of a single 

 year a small quantity difficult to appreciate, but various disturbing 

 causes, of which the splitting of the bark and its falling off in 

 scales and the varying amount of moisture in it, are the chief, 

 combine to mask or exaggerate it, as the case may be. The mea- 

 surements should betaken each time along the same circumference; 

 and in order to secure this each stem should be encircled with a 

 steady line of white paint about half an inch wide. The most last- 

 ing paint is zinc white. According to the nature of the bark, the 

 ring of paint must be renewed at longer or shorter periods. The 

 entire ring should lie in the plane at right angles to the axis of the 

 bole at the height at which it is painted. The rings should be 

 painted at 4i feet from the ground, but if there is some marked 

 irregularity of growth at this height, two rings may be painted 

 respectively above and below this height at an equal distance from 

 it in accordance with the principle laid down in rule (/) on page 



