62 INSTRUCTIONS FOR MAKING FOREST SURVEYS, ETC. 



The aneroid barometer is often used to determine 

 the distance in elevation between the ends of the pro- 

 posed trail, and the approximate distance may be de-' 

 termined by pacing. This furnishes a preliminary 

 reconnaissance. A " trial" or "random" line may then 

 be run from one end of the proposed line to the other 

 on the approximate average grade, which has been de- 

 termined by reconnaissance. This may be done by a 

 grademeter, an Abney level, or a Locke level. 



The grademeter is used as described on page 29. 

 As the circular pendulum is graduated to tangents it 

 may be used to line in the grade to any desired per cent, 

 either uphill or downhill. It is unnecessary to consider 

 the matter of distance, because grade, as thus meas- 

 ured, is an absolute quantity in itself. 



The Abney level is used in a similar manner, but it 

 contains no swinging pendulum, and must be set to the 

 desired grade before the sight is taken to the instru- 

 ment. Some of the. Abney levels are graduated to 

 degrees ; others to degrees and slopes, in the proportion 

 of 1 : 1 and 1 : 10; others have graduations for per cent. 

 This has led to some confusion, and some bad construc- 

 tion has resulted. Care should be used to apply only 

 the per cent when this instrument is in use on trails. 



The Locke level is a simple hand level which does not 

 sight either uphill or downhill ; it is used by sending an 

 assistant ahead with a pole, upon which sights are taken 

 through the barrel of the level. Allowance must be 

 made for the height of the surveyor's eye above the 

 ground. Thus, if his eye is 5 feet above the ground 

 he can fix the location of a 5 per cent grade by working 



