
INSTRUMENTS USEFUL TO A WOODSMAN. 139 
meeting at a zero point which is intersected by a line passing 
through the longitudinal center of the slide. The right-hand 
scale runs to 75 and the ieft-hand scale to 225, the latter scale ex- 
tending upward on the left-handend barofthe frame. The right- 
hand crossbar is provided with a vertical scale running upward 
from zero to 100, and continued on the left-hand crossbar with 
a scale running upward to 175. These scales are divided in fifths 
and numbered. The lines forming the scales are equally sepa- 
rated from each other and represent units of distance under any 
system of measurement that may be adopted. The handle of the 
device is attached to the left-hand crossbar. 
To use the instrument, the observer measures the horizontal 
distance in feet, yards, or in any other desirable unit, from where 
he is to stand to the base of the tree. He then sets the slide by 
one or the other of its two index marks, which is brought into line 
with the graduation on the vertical scale corresponding to the 
measurement just secured. If the distance is less than 75, the 
slide should be set so that the upper end of the plumb line will 
take a position opposite the required number on the portion of the 
vertical scale on the right-hand crossbar. If the distance is more 
_ than 75, the slide should be pulled out and reversed end for end 
and adjusted until the index mark at its then lower end is brought 
opposite the required number on that portion of the vertical scale 
on the left-hand crossbar. The observer then looks through the 
eyepiece and objective and brings the hair of the latter into line 
with the top of the tree. The plumb line is allowed full play and 
crosses the left-hand horizontal scale. As soon as the plumb line 
is at rest the number which it crosses is read off in the mirror. 
This number indicates the height of the tree from the level of the 
observer’s eye to its top. He then sights through the instrument 
to the base of the tree and reads the number crossed by the plumb 
line on the right-hand horizontal scale. This number indicates 
the distance from the level of the observer’s eye to the base of the 
tree, and is added to the number before secured, which gives the 
total height of the tree. If the observer should be standing so 
that the level of his eye is below the base of the tree, he should 
first determine the height from the level of his eye to the top of 
_the tree, then the height from the level of his eye to the base of 

