248 THE MEASUREMENT OF STANDING TREES 



as it does on most dendrometers, the proportion between the upper diameter B and 

 its equivalent C, corresponding to c on the instrument, is altered since the side ^46 

 remains of the same length and coincides with Ab2 in the figure. This discrepancy 

 increases in proportion to the cotangent of the angle A and the distance read on the 

 dendrometer scale at 62, which is graduated for inches, will be less than the true 

 diameter B by just the amount of this error. The use of all dendrometers built 

 on these principles requires correction by a table, to obtain true upper diameter. 



This difficulty is illustrated by a dendrometer attached to the Barbow cruising 

 compass, used to some extent on the Pacific Coast. The dendrometer on this 

 compass was a brass scale 1 inch long, finely graduated to read the apparent diameter 

 in inches at the upper end of the desired log, when held exactly 1 foot from the 

 eye by means of a string. But the true diameter had then to be looked up in a 

 table furnished with the compass. The correction varied with the angle of sight; 

 that is, with the number of log lengths in the tree. All readings were made at 

 100 feet from base of tree. 



On the Pacific Coast a second plan has been adopted, that of making the length 

 of the scale 6i equal to the diameter B, thus substituting two parallel lines of sight 

 for the horizontal triangles shown, and reading the diameter of the lower side of a 

 parallelogram directly in terms of inches of diameter at B. In an instrument 

 invented by Judson F. Clark and C. A. Lyford, a telescopic sight moves on a bar. 

 In one invented by Donald Bruce, both lines of sight are brought into the same 

 plane by means of two reflecting mirrors, set at exact angles of 45 degrees. 



201. The Biltmore Pachymeter.' By employing the second principle, in which 

 the side of the small triangle biC remains vertical, the diameter indicated at bi 

 on the hypsometer remains in the same proportion to that desired at B, as when 

 the reading is taken at position C. Since the point opposite c may be taken at 

 the base of the tree, regardless of whether this point is horizontally opposite the 

 eye or above or below it, a projection of the diameter B upon the base of the tree 

 enables it to be directly measured on the tree, or on a scale c upon the instru- 

 ment, graduated for the distance Ac. This principle is employed by an instrument 

 termed the "Biltmore Pachymeter." (Ref. Forestry Quarterly, Vol. IV, 1906, 

 p. 8.) A slot, the two edges of which are absolutely parallel, or a stick or cane 

 of which the same is true is suspended in a vertical position in front of the eye. 

 A scale marked in inches is held by an assistant tangentially to the tree trunk at 

 D.B.H. The diameter at any desired point on the stem is obtained by finding the 

 distance from the tree at which the diameter of the slot or stick exactly obscures 

 that of the tree at the desired point, when the width corresponding to this diam- 

 eter will be indicated by the intersections of these edges on the scale below. The 

 instrument and its projection upon the tree trunk are shown in Fig. 54. 



202. The d'Aboville Method for Determining Form Quotients. This method 

 depends on the measurement at 62, but is simplified by using a horizontal line of 

 sight from eye to tree, and an angle of 45 degrees at point A, in which case the 

 proportion between the lines AC and AB in Fig. 54 becomes 1.4, and the diameter at 

 B becomes 1.4^2. To make this measurement, a distance is found which is just 

 equal to the length of the bole between the point horizontally opposite the eye, as 

 in Fig. 54, and the upper point to be measured. 



Substituting d and D for diameter at | height and D.B.H. respectively, the 



form quotient of a tree, as read on the dendrometer, is 



d 

 / = -Xl,4. 



• Pachymeter — an instrument for measuring small thicknesses. — Century Dic- 

 tionary. 



