472 



Mr. B. Stewart on certain Eccperiments 



[June 18, 



XV. ^^An Account of certain Experiments^ on Aneroid Baro- 

 meters_, made at Kew Observatory, at the expense of tlie 

 Meteorological Committee."'' By B. Stewart. Communi- 

 cated by the Meteorological Committee. Received June 3_, 

 1868, 



In judging of the value of an instrument, 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 us ; but it is rather the constancy of its 

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

 that determines its value. 



An aneroid may differ from a standard barometer at tbe ordinary 

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

 ferences can be well ascertained and remain constant, such an instrument 

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

 constancy, but of which the rate is wrong. 



The circumstances which may be 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. — Of the influence of time, I am not able to say much ; Captain 

 Henry Toynbee has allowed me to examine the various readings of an 

 aneroid, which he carried about with him for many years in his voyages, 

 and constantly compared with a standard barometer. 



This aneroid (which I shall call No. 1) was between 4 and 5 inches in 

 diameter, and was compensated for temperature. 



In July 1860, as compared with a standard barometer, it read 0-025 in. 

 too low. In September 1862 it read (at the same temperature) about 

 0*012 in. too low; while in March 1864 (still at the same temperature) 

 it read about 0*020 in. too low. 



This instrument, which was well cared for, and which, being used chiefly 

 on the surface of the ocean, was subjected neither to a very great nor to a 

 very sudden change of pressure, must be allowed to have retained its cha- 

 racter with great constancy. 



This is the only definite information regarding the effect of time on these 

 instruments which I have received. 



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

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

 extent such compensations are trustworthy. I record the results (obtained 

 at the Kew Observatory) of subjecting six aneroids, each 4^ inches in 

 diameter, made by two different makers, to a very considerable range of 

 temperature. 



