The Abney Hand Level. 349 



with the aneroid compare favorably with bench marks and the 

 resultant accuracy is well within the limits of the precision of 

 the horizontal control. 



In order to secure a consistently accurate topographic map of 

 greatest practical value to the timber appraiser, logger, trail and 

 road builder, and forester, the conditions affecting the work 

 with the aneroid should be studied, with the end in view, that 

 some other method of obtaining vertical control may be substi- 

 tuted at the time when local factors affect the work with the 

 aneroid so unfavorably as to destroy the value of the results 

 for the construction of the type of forest map desired. 



The Aneroid Barometer. 



The aneroid is not always an accurate instrument and the 

 errors encountered in working with the aneroid can hardly be 

 controlled by the man in the field. The elevations arc not secured 

 by geometric principles. The levels are determined by an intricate 

 mechanism which measures the weight of the column of air 

 pressing upon the top of a shallow cylindrical box. The top 

 is composed of corrugated metal so elastic as to respond to 

 changes in pressure. The interior of the box is in vacuum. 

 When the atmospheric pressure decreases the elasticity of the 

 corrugated top presses it outward, and when the atmospheric 

 pressure increases the top is pressed inwards. This movement 

 of the corrugated top is communicated to an indexed dial by 

 means of a complex system of multiplying levers, chains, and 

 springs. The possibilities for error in the mechanism of such 

 an instrument are apparent. No system of office corrections 

 will compensate for them. The errors caused by the daily and 

 hourly changes in atmospheric pressure can be eliminated by 

 determining these changes with a stationary barometer in camp 

 and correcting the elevations taken during the day in the field. 

 This, however, can only be done with an accurate camp baro- 

 meter, preferably a mercurial barometer. Two aneroids read 

 in camp will often vary considerably even if not moved and it 

 can not be determined which is the more accurate. The errors 

 peculiar to each instrument in the field however, can not be de- 

 tected and remain undiscovered. Errors in the aneroid readings- 



