45, CORNHILL, B.C., AND 122, REGENT STBEET, W., LONDOX. 295 



FIG. 1179. 



ANEROID BAROMETERS FOR MEASURING ALTITUDES 

 OR SURVEYING. 



1180 Owing to the inconvenient size of Mercurial Standard Barometers, and also 

 from the great risk of breakage in transit, it often occurs that their use has been 

 abandoned by surveyors where otherwise they would have been invaluable for strict 

 altitude measurements. 



This difficulty is almost entirely overcome by the use of Negretti and Zambra's 

 Altitude and Surveying Aneroid Barometers. These instruments are now constructed 

 with such precision that very small elevations may be ascertained with great exacti- 

 tude. The scale of the altitude aneroid is laid off by actual experiment in a vacuum 

 chamber, the readings being noted both backwards and forwards, such readings 

 being repeated at long intervals, and the observed differences corrected, before 

 finally dividing the scale on the dial. Several tests are also applied to the instrument 

 in order to compensate for errors arising from varying extremes of temperatures. 



Where it can be conveniently carried, Negretti and Zambra would in all cases 

 recommend their full-sized Altitude Aneroid (fig. 1180) for observers' use ; as from 

 the large diameter of the divided circle, exceedingly minute movements of the 

 index hand may be seen with ease. 



