MEASUREMENT OF HEIGHT. 



93 



while a reading is being made. The board being adjusted so that the 

 top line Aa meets the top of the tree C (or the " timber-height " of the 

 stem, if this is being measured), then, in the similar right-angled triangles 

 ABC and a b c, B C : b c = A B : a 6. As AB has been measured and 

 a b corresponds with this distance, the reading b c will at once give B C, 

 to which, if the observation be made on level ground, must be added the 

 height from the ground to the eye of the observer. On hillsides a double 



reading may be added to or 



Fig- 22. subtracted from the original 



observation, according to the 

 observer's position. 



For example, if A B, 

 measured, is 70 ft. ; then, 

 when Aa has been adjusted 

 in line with C, the reading 

 be (b being point 70 on the 

 side scale) will correspond 

 with B C, and can be read 

 off as d e on the scale along 



the base of the board. If d e ( = b c) is 45 ft. , 

 then B C is 45 ft., and the total height of the 

 tree (B C and B D) will be 45 ft. plus the dis- 

 tance of the observer's eye above the ground. 

 f Faustmann's Mirror- Hypsometer (Fig. 22) 



is on the same principle. The distance of the 



observer from the tree having been measured, the movable upright arm 

 is set accordingly. When short (0-15 yards, or multiples of 0-15 ft.), the 

 end of this marked II. is adjusted on the right-hand scale, to form the 

 similar triangle ; while for longer distances (15-30 yards, or multiples of 

 15-30 ft.), the end marked I. is set against the scale on the left-hand 

 side representing the number of yards (or multiples of feet) between 

 the observer and the stem. The observation is made through an aperture 



