374 Forestry Quarterly. 



takes readings at the base and the top of a tree on the opposite 



bank. The first reading at the base of tree is 2%, the reading at 



top of tree is 20%. The vertical angle then is 18%. Arriving 



at the opposite side the height of the tree is measured and found 



to be 150 feet high. For every 100 feet of horizontal distance 



150 

 there is a vertical distance of 18 feet, therefore — tt- x 100=8^'?. 



18 '^'^ 



distance across the canyon. 



The difference of elevation between the two banks is .02 x 833 

 or 16.6, or 15 feet expressed to the nearest five feet. Since the 

 observation was taken at the base of tree 5 feet (height of eye 

 above ground) should be added to this reading. 



The above method of determining distance was employed many 

 times last winter and it works out beautifully. 



While aneroid barometers with the most careful use and careful 

 corrections were daily giving us errors of 100 to 200 feet, our re- 

 sults from the use of the Abney level were checking within 5 and 

 10 feet. Occasionally, one makes an error of 20 feet in a two- 

 mile strip. The greatest error made was 35 feet in running a dis- 

 tance of 2-| miles. Such small errors are easily distributed. 



Every individual would not be adapted to use the Abney hand 

 level with practical success. Curious as it may seem, those that 

 are inclined to be too precise and fussy often make the biggest 

 errors. The compassman using the Abney must keep ahead of the 

 cruiser ; he can not take time to make a second observation, he can 

 not aflford to lose his object sighted at, he can not consume extra 

 time in making more close computations than are necessary, and 

 he must learn what features of topography to omit. Numerous 

 short sights requiring reference to the table frequently should be 

 avoided. One learns to estimate small differences in elevation 

 to the nearest 5 feet and it does not add to the accuracy of the 

 work to take observations for short distances involving small dif- 

 ferences of elevation. 



In going through patches of dense, tall reproduction where 

 long sights can not be secured one learns to estimate the diflference 

 in elevation as he goes along and checks this by estimating the 

 average per cent of slope. After a few days' practice it is sur- 

 prising how close one checks up on this work. 



Often just before reaching a big patch of brush or reproduction 



