438 
MR. BAILY’S DESCRIPTION OF A NEW BAROMETER. 
liable to expand and contract with different degrees of moisture, independent of the 
temperature, no correction for this purpose can be depended upon. This is a great 
imperfection in the mode of constructing and fixing the scale of a barometer in- 
tended for very accurate purposes. The specific gravity of the mercury was ascer- 
tained by Mr. Faraday to be 13 - 624: the thermometer being at 40°, but the height 
of the barometer not given. 
I have already stated that prior to the year 1823, the registers do not indicate 
whether the observations have been corrected, or not ; but that, commencing with 
January 1823, they profess to give the corrected heights of the readings of the baro- 
meter, unexplained however as to the mode of correction. This continued till 
March 20, 1826, when a temporary suspension of the observations took place. From 
April 6, 1826, down to the end of the year 1836, we are again left in doubt whether 
the daily observations are corrected, or not. But the inference is that they were not 
corrected ; since we find a correction applied to the monthly means, for temperature 
and capillarity. I have ascertained, however, on inquiry, that the daily observations 
have in all cases been partially corrected : that is, the correction for the capacity of 
the cistern has been applied daily. Why this correction alone, on each day, should 
have been considered requisite, 1 have not been able to ascertain ; and as it is nowhere 
mentioned in the meteorological journal, it may perhaps have sometimes led to error. 
But leaving this part of the subject, I shall now proceed to notice the loose manner 
in which the remaining correction (for temperature) has been from time to time ap- 
plied to the monthly means. 
From April 6, 1826, to the end of that year, the temperature has been taken from 
the external thermometer, instead of the thermometer which dips into the cistern of 
mercury. Consequently all the reduced values of the readings are too great. By the 
external thermometer, I mean the thermometer which is placed outside of the building, 
and consequently gives the temperature of the open air. 
In the year 1827 this error appears to have been discovered and discontinued; but 
another of a different nature was at the same time introduced. For, from that epoch 
to the end of the year 1836, all the corrections are made under the assumption that 
the height of the mercury in the barometer was exactly 30 inches : when it is well 
known that the correction will vary according to the variation in the height. In fact 
there does not appear, at any time, to have been any regular and uniform system of 
reduction adopted. 
Now this state of confusion and uncertainty ought not to exist in a meteorological 
journal emanating from this Society, more especially as the true values are as easily 
attainable as the approximate ones. And although, in a general point of view, the 
minute differences caused by such errors may be unimportant, yet as appeals are fre- 
quently made to the barometer of this Society, as a standard, by persons engaged in 
important researches, the most scrupulous accuracy ought to be adopted and pursued, 
and the fullest explanation placed on record. And notwithstanding the details which 
