SAMPLE PLOTS. 97 



permissible), takes the mean cubic contents of these true 

 average stems and multiplies this by the total number of 

 stems in the crop or the sample plot to find total cubic con- 

 tents. And of course in the case of a sample plot the result 

 is multiplied in due proportion to give the cubic contents for 

 the whole crop. 



This measurement by sample plots presumes that each crop is 

 sufficiently regular to permit of sample plots being taken so as 

 to give a correct average for the whole crop or wood. In 

 Britain it is usually extremely difficult to select true average 

 sample plots, owing to woods having generally been over- 

 thinned and often also damaged by wind ; for a true result 

 can only be obtained when the sample plots represent a true 

 average of the crop. But wherever the crops are growing 

 regularly and in fairly close cover, this is the best method of 

 estimating the cubic contents of mature or maturing crops for 

 the purpose of making a working -plan. In pole-crops 1-in. 

 diameter-classes are most convenient, and for large tree-crops 

 2 in. diameter- classes, the poles in each 1-in. class including 

 all those, say, from 4J in. to 5J in. as 5 in., 5J in. to 6J in. 

 as 6 in., and so on; and here also, to eliminate errors of 

 eccentricity, the stems should be measured alternately 1ST. and S. 

 and E. and W., if callipers be used ; while if an oiled linen or 

 a steel tape be used for girthing, it should be pulled from, side 

 to side to get it as near truly horizontal as possible. 



On the Continent the cubic contents of woods of different 

 ages can also be estimated fairly accurately from Average 

 Yield Tables, giving the average cubic contents of similar crops 

 that have been carefully measured after felling. These tables 

 are framed for different kinds of timber and different qualities 

 of land ; but except as indicating what can be grown on the 

 Continent under good management, they are of little or no 

 practical use for Britain, and therefore need not here be 

 referred to in detail. 



