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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 the 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 may be supposed to affect the indica- 

 tions of an aneroid may be classed under three heads, namely : — 



(1) Time. 



(2) Temperature. 



(3) Sudden variations of pressure. 



(1) Time. — Of the inSuence of time I am not able to say much ; 

 Captain Henry Toynbee has allowed me to examine the various read- 

 ings 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 character 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 in- 

 vestigated 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. 



