The Abney Hand Level. 351 



rupt and steep slopes are unfavorable because it has been found 

 that the aneroid is most liable to show instrumental error when 

 there is a sudden change in pressure under which conditions the 

 aneroid is not able to respond as quickly as is required. If there 

 are abrupt changes in elevation and slope the aneroid will act 

 sluggishly and the first reading will not be accurate. The com- 

 passman must wait a few minutes and allow the instrument to 

 settle. Where these abrupt descents and ascents are made fre- 

 quently much time will be lost in waiting for the aneroid to 

 read correctly. Herein lies the objection to the use of the 

 aneroid in rough and rugged country. Where the change in 

 elevation is gradual the instrument adjusts itself to the change 

 in atmospheric pressure while the compassman paces along the 

 line. No time is lost then in waiting for the aneroid to read 

 correctly and large instrumental errors are not so apt to occur. 

 This is the case in uniformly sloping country where the slopes 

 are rounded and the grades are not changed abruptly at definite 

 points. Such country is most favorably adapted to the use of the 

 aneroid. 



2. In country where the strips can be run with some speed and 

 where it is possible for the compassman to finish the mile be- 

 tween base line benchmarks within a comparatively short time 

 the aneroid can be used satisfactorily. Frequent comparisons 

 with known elevations will thus compensate the errors in the 

 readings of the aneroid. Wilson on this point says: — "Where 

 the changes in slope are not great and the journey is made with 

 considerable speed and when the time consumed in travel is 

 comparatively short, the aneroid may safely be used for dis- 

 tances as great as three to five miles though in such cases the 

 aneroid may not check out within a contour interval on the 

 next comparison." In smoothly sloping country where the topo- 

 graphy is not broken the compassman can work with greater 

 speed than where the topography is rough and rugged. As a 

 consequence he will be able to compare his aneroid readings with 

 base line benchmarks within short intervals of time. 



However, the speed with which the compassman can run the 

 strip is not the only factor in determining the working speed of 



