350 Forestry Quarterly. 



often become obvious to the compassman and he has no means 

 of accurately correcting them. 



The instrumental errors of the aneroid are outlined as fol- 

 lows in Wilson's book on Topographic Surveying: — 



1. The elasticity of the corrugated top of the vacuum chamber is 

 affected by rapid changes in pressure. 



2. The readings are affected by changes in temperature which it is 

 impossible to readily compensate. 



3. The different spaces on the scale are seldom correct relatively one 

 to the other, but the scale of pressure in inches is more accurate than 

 the scale of feet since the latter contains the error due to the formiplae 

 by which it was graduated. 



4. The weight of the instrument affects its indications, its readings 

 differing in accordance with the position in which it is held. 



5. It lacks in sensitiveness frequently not responding quickly to changes 

 in altitude. 



6. The chains and levers sometimes fail to quickly respond to the 

 movement required of them. 



7. Because of its containing so many mechanical parts these are sub- 

 ject to shifting or jarring by movement made in transporting it, the 

 only remedy for which is frequent comparison with known altitudes or 

 a mercurial barometer/' 



It is readily understood that most of these errors are different 

 for each instrument in the field and that no office correction can 

 eliminate them. No corrections based on camp readings can re- 

 move the errors in the field barometers caused by other factors 

 than atmospheric changes. The only possible method of cor- 

 recting or eliminating these other errors of the aneroid is in 

 the field by the compassman. The compassman must study the 

 cause of these errors and must not rely too much upon the camp 

 corrections applied by the camp draftsman. 



In order to get the best results with the aneroid the instru- 

 ment must be handled carefully and intelligently. The errors 

 introduced by changes of conditions must be recognized by the 

 compassman and topographer. Ignorance of all the factors 

 influencing the accuracy of the results of the use of the aneroid 

 will depreciate the value of the map, and this depreciation will 

 not be due to any defect of the instrument itself. Understanding 

 clearly the cause and nature of the errors encountered it is pos- 

 sible to draw some conclusions as to the conditions under which 

 the aneroid is most favorably used and to determine approxi- 

 mately under what conditions the aneroid is an unsuitable in- 

 strument. 



Conditions Favorable to the Use of the Aneroid. 

 I. The change in slope or topography must be gradual. Ab- 



