MULL. METEOROLOGY. BAROMETER. 547 



While on this subject I must mention some facts which 

 have occurred to me on comparing the movements of dis- 

 tant barometers ; having been led to observe them from 

 attempting to ascertain altitudes by referring, as a standard, 

 to a fixed instrument. They are interesting, both as they 

 relate to the history of the atmosphere and to that of the 

 instrument. They are not less so in a practical view ; as 

 they inculcate the necessity of precaution in estimating 

 altitudes by the barometer when the horizontal distance 

 between the upper and lower station is considerable ; as 

 well as that of great care in the construction or choice 

 of the instrument itself. 



It is well known that the oscillations of the mercurial 

 column become greater progressively as we increase our 

 latitude ; bat that, with this difference, there is a general 

 conformity in them in different latitudes : in Europe at 

 least. But it has also appeared, in comparing the obser- 

 vations above alluded to, that at different stations, and 

 independently of change of latitude, or of the effects 

 arising from it, the variations of any two or more barome- 

 ters are not always the same in quantity, and do not occur 

 at the same time : they are neither equal nor simultaneous. 

 With the intention of verifying the nature and extent of 

 these inequalities, various registers were compared ; all 

 the instruments being of the best construction, and the 

 observers of unquestionable accuracy. For the purpose 

 of estimating certain errors arising from imperfect con- 

 struction, hereafter to be mentioned, a parallel set of 

 experiments were conducted with a number of barometers 

 in one place ; some of them being purposely made in 

 different ways imperfect. No other mode of estimating 

 errors from this cause is possible ; since, had any of the 

 barometers at the distant stations been approximated and 

 compared, it might still be suspected that they had under- 

 gone some change after separation. I think it proper 

 however to remark, that, among these comparative expe- 

 riments, different attempts were made to derange the 



