432 
MR. BAILY’S DESCRIPTION OF A NEW BAROMETER. 
imperfection in either of the tubes is immediately detected on comparison with the 
other. And, considering the care that has been taken in filling the tubes, it may 
justly be considered as a Standard Barometer. The specific gravity of the mercury 
was determined by Dr. Prout to be 13*581 ; the thermometer being at 62°, and the 
barometer at 30 inches*. 
The second part of the present volume of the Philosophical Transactions will con- 
tain the first register of the observations that have been made with this instrument. 
The daily observations are recorded just as they are read off from the scale, without 
the application of any correction whatever. This will be found, on due consideration, 
and after the details which I shall presently state, to be the most simple, and by far 
the safest plan of registering them ; whatever mode may be afterwards adopted of re- 
ducing and discussing them. At the end of each month the uncorrected mean is de- 
duced ; which mean, however, will also be given corrected agreeably to the usual 
formulae, to which I shall now proceed to advert. 
The observed height of the mercury in a barometer requires several corrections 
(differing according to the construction of such barometer) in order to determine its 
absolute height, or that point when it may be considered strictly comparable with 
another barometer, either of the same or of a different construction : and, for effecting 
this end, certain conditions are previously understood, and universally assented to. 
Thus, the temperature of the mercury is always supposed to be at the freezing point 
of water, or 32° Fahrenheit : the scale, by which the height is measured, if liable to 
expansion by heat, is always reduced to the standard temperature, which in this 
country is 62° Fahrenheit : the tube must be corrected for its capillary attraction : 
and lastly, proper allowance should be made, in certain cases, for the elevation of the 
place of observation above the mean level of the sea. I shall speak of each of these 
in their order-f-. With these corrections duly made, the absolute heights of two baro- 
meters might be considered comparable with each other, although separated by the 
whole diameter of the globe : and with barometers, formed of tubes of a considerable 
diameter, and having a well adjusted scale, this is probably the case. Yet as, even 
in the best barometers, there are still certain sources of discordance, some of which, 
although slight, cannot be altogether avoided notwithstanding our utmost care, such 
as differences in the specific gravity of the mercury, or in setting off the measure of 
the scale, or an uncertainty in the height of the station above the mean level of the 
sea, and, in the more usual ones, others of a more formidable and variable nature, 
depending on circumstances not yet sufficiently accounted for, it is always the most 
* Dr. Prout has been good enough to inform me that, in taking the specific gravity of mercury in the common 
mode, it is necessary, in order to expel the whole of the adhering air, to heat repeatedly the mercury in the 
vessel to nearly the boiling point, and in this state to expose it under the exhausted receiver of an air-pump. 
This precaution was taken in the present instance. 
f In those barometers where the tube dips into a measured cistern (similar to that which was constructed 
for this Society by Mr. Daniel, to which I shall presently allude) there is another correction requisite, which 
depends on the relative capacity of the tube and the cistern : but this does not apply to the present barometer 
