442 DE. C. CHRBB, EXPERIMENTS ON ANEROID BAROMETERS 



pressure being reduced at the rate of 1 inch in 3 or 4 minutes, readings have been 

 taken of the aneroid and a mercury gauge attached to the pump. The readings are 

 taken with the pressure temporarily stationary, usually for each inch, but sometimes 

 for each half-inch of pressure. After the reading has been taken at the lowest point 

 for which verification is desired the pressure is reduced a very little further, and then 

 maintained constant for some minutes. It is then allowed to rise by the readmission 

 of air, stoppages being made and readings taken during the ascent of pressure 

 precisely as during the descent. 



From the observed differences between the aneroid and gauge the readings of 

 the latter being corrected of course for temperature corrections are calculated 

 separately for the descending and ascending readings of the aneroid, and these are 

 given on the certificate issued. 



If the aneroid possesses, as is most usual, a scale of altitudes in feet or metres, 

 that is read as well as the pressure scale, and corrections are given to it based on 

 Airy's table connecting altitude and barometric pressure. Whether the unrestricted 

 application of Airy's table is the best means of interpreting observations taken during 

 mountain ascents is an interesting question, but I do not propose to discuss it here, 

 confining my attention to the aneroid as a measurer of pressure. 



Aneroids intended for the Meteorological Office are usually constructed only for 

 the narrow range, 31 to 26 inches. In their case there are exact regulations as to 

 the size of error permissible and other points, and the issue of a certificate implies 

 the attainment of a certain standard of excellence. With this exception, however, 

 there exist no rules for rejection, and the certificate merely represents the report of 

 an expert on an instrument submitted to him. The owner is supposed to draw his 

 own conclusions from the figures submitted. This view of a certificate was apparently 

 that originally dominant at Kew Observatory, but of late years there has been an 

 increasing tendency on the part of the public to substitute the idea of a " certificate 

 of excellence" for that of a " certificate of examination." What is sought is protec- 

 tion against inferior instruments rather than the means of applying corrections to 

 observed readings. It has thus become desirable to ascertain how the excellence of 

 an aneroid may be judged of, and one of the chief objects of the present investigation 

 was to obtain data suitable for the purpose. 



General Character of Phenomena. 



2. An aneroid is usually graduated by direct reference to a mercury gauge during 

 a reduction of pressure. An examination, made under the same conditions as the 

 graduation, tests the accuracy of the workman, but not necessarily the quality of the 

 instrument. That something more is desirable becomes obvious, when, after reducing 

 the pressure several inches, one keeps it constant and continues comparative readings 

 of the gauge and aneroid for some time. A gradual fall in the readings of the latter 



