THE ANEROID BAROMETER 



109 



at 5 P. M. it was 29.100. That difference in pressure 

 corresponds to nearly 150 feet in elevation, and height 

 observations made during the day would be uncertain to 

 very wide limits if the change could not be allowed for. 



THURSDA Y FRIDA Y 



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The possibility of correction rests in two suppositions: 

 (1) that at any moment of time the air pressure is constant 

 over a considerable horizontal area, and (2) that the field 

 barometer and the station barometer work together, and 

 that they both follow exactly and quickly the change of air 

 pressure. The latter point may be expressed in this way 

 that the field barometer, if left at the base station, would 

 have followed the same course as did the instrument which 

 in fact was left there. 



The field barometer may not read the same as the 

 barograph when they are brought together, but that 

 " index error," as it is called, does not matter if the differ- 

 ence between the two remains constant. In this case the 

 field barometer at camp in the morning read 29.350 and at 

 night 29.200, .1 inch higher than the barograph. One 

 may, therefore, when he gets to computing, draw on the 



