436 
MR. BAILY’S DESCRIPTION OF A NEW BAROMETER. 
Or, in other words, the height of the mercury in the barometer would in such case 
be 30-1045 inches, if placed at the mean level of the sea, instead of being in the apart- 
ments of the Society: and so in the proportion of -0011 inch for every foot below its 
present position. But, as I have before remarked, this correction is wholly omitted 
in the Meteorological Journal. 
I have been particular in giving these explanations as to the precise mode in which 
the corrections should (and are now directed to) be made, since it appears that great 
irregularity, as well as some inattention, error, or confusion has hitherto occurred on 
this subject, which ought not to have existed ; and the Meteorological Journal of this 
Society has lost much of its utility, confidence, and importance in consequence 
thereof. 
Prior to the year 1823, the registers of the barometer do not indicate whether the 
observations are corrected or not : nor can I obtain any satisfactory information on 
this point. So that a person now referring to them can consider them only as ap- 
proximate values. The barometer then in use is still in existence. 
In January 1823 the registers commence (as I presume*) with the new barometer 
which had been constructed in the preceding year under the able direction of Mr. 
Daniel, now Professor of Chemistry at King’s College. A description of this baro- 
meter is given by him in his Meteorological Essays and Observations, page 353. The 
daily observations are, in the register, said to be corrected ; but no formula or rule is 
given, of the mode in which the corrections have been made: and if the observations 
have been corrected by the small table engraved on the face of the barometer (which 
is the same as that given by Mr. Daniel in page 372 of his Essays ), the result will in 
most cases, for the reasons which I shall presently mention, be slightly erroneous ; 
but more so as the temperature varies from the freezing point. So that although, 
during the winter months, the results will not be far from the truth, yet in the sum- 
mer they will not exhibit the correct values-f-. For, that table has been calculated 
“ from the expansion of mercury and mean dilatation of glass :” it having been origin- 
ally intended (as I have understood) that the divisions of the scale should have been 
cut on the glass tube. But this plan having been abandoned, and recourse had to the 
ordinary mode of construction, it is evident that the expansion of the glass tube does 
not affect the observed height of the column of mercury sustained by the atmosphere. 
The only effect which the expansion of the glass can have on the reading of the 
vernier, will be caused by an alteration in the relative capacity of the tube and the 
cistern ; but this would be so extremely small, on all ordinary occasions, as to be 
* There is nothing stated in the register by which we can judge whether the old barometer, or Mr. Daniel’s, 
was at that time used for the daily observations ; except that the height of the cistern of the barometer is then 
stated to be 19 feet higher than before : which was the position in which I find that Mr. Daniel’s barometer 
was placed, as I shall presently explain more fully. 
f Taking the thermometer at 70°, and the barometer at 30 inches, the true correction would be •114; but, 
according to the table attached to the barometer, it is only '098 : being a difference of '016 inch. 
