EXPERIMENTS ON ANEROID BAROMETERS MADE AT THE KEW 



OBSERVATORY. 



By B. Stew.\rt, LL. D., F. E. S. 



( From the Proceedings of the Royal Society, London. ) 



[The frequent use of the aneroid barometer in meteorolof^ical observations and in topo- 

 p'aphical surveys in this country will render the following paper from the proceedings of the 

 Royal Society of London interesting to many of the readers of the Smithsonian Report. — J. H.] 



In judging' of tlie value of an instrument, says tlie report, such as an aneroid, 

 it is not the mere extent of difference between its indications and those of a 

 standard barometer that ought to guide ns; but it is ratlier the constancy of its 

 indications under the various circumstances to which it may be subjected, that 

 determines its vahie. An aneroid may differ from a standard barometer at tlie 

 ordinary pressure, and to a greater extent at other pressures ; but provided these 

 differences can be well ascertained and remain constant, such an instrument ought 

 to be regarded as valuable, just as much as a chronometer of known constancy, 

 but of which the rate is wrong. 



The circumstances which maybe supposed to affect the indications of an aneroid 

 may be classed under three heads, namely : 



1. Time. 



2. Temperature. 



3. Sudden variations of pressure. 



1. Time. — As to the influence of time, no definite experiments were made. 



2. Temperature. — A good aneroid is generally compensated by its maker fi)r 

 the effects of temperature, and the question to be investigated is, to what extent 

 such compensations are trustworthy. I record the results of subjecting six ane- 

 roids, each four and one-half inches in diameter, made b}^ two different makers, 

 to a very considerable range of temperature. 



These results are, on the Avhole, very satisfactory, and appear to show that a 

 well-made compensated instrument has its indications comparatively little affected 

 by a very considerable temperature cliange. It ought always to be borne in mind 

 that an aneroid is not capable of being read to the same accuracy as a standard 

 barometer, and that the 1-lOOth of an inch is a very small quantity. These 

 temperature experiments were made at the ordinary atmospheric pressure. I am 

 unable to say what effect a change of temperature would have at a diminished 

 pressure. 



