INSTRUMENTS USEFUL TO A WOODSMAN. 135 
In laying off a square the cruiser first runs out one side of it 
and then takes a position at the point where he wishes to deter- 
mine a right angle. With the instrument in one hand he then 
looks through one of its sight openings at an object in the prede- 
termined line, such as stake or pole. An assistant in the approxi- 
mate location of the desired new line now moves about until his 
| image appears in the mirror below the sight opening being used 
and exactly in line with the stake or pole. The assistant is then 
standing in a line ata right angle to the predetermined line and 
establishes the new line. 
| The box of the instrument shown in the plate is about an inch 
| and a half long and about an inch and a quarter high, and with its 
| handle fits in a small box. Cost, $4. 


INSTRUMENTS FOR MEASURING HEIGHTS. 
There are several methods of determining the height of a stand- 
ing tree. One of the simplest is to measure the shadow of the 
tree and the shadow of a straight pole of known length set per- 
pendicular to the earth. Multiply the length of the shadow of 
_the tree by the length of the pole and divide the product by the 
length of the shadow of the pole. The result will be the height 
of the tree. 
A method used when the sun is not shining is to set-two poles 
in a line with the tree. (See fig. 7.) From a point on one pole 
sight across the second pole to the base and to the top of the tree. 
Let an assistant note the points where the lines of vision cross the 
second pole and measure the distance between these points. Also 
measure the distances from the sighting point on the first pole to 
the base of the tree and to the lowest vision point on the second 
pole. Multiply the distance between the upper and lower vision 
points on the second pole by the longer of the other two measure- 
ments and divide by the shorter; the result will be the height of 
the tree. 
Example: Let ab=6; Sb=4; and SB=30; then See 
height of tree. 
