MR. BAILY’S DESCRIPTION OF A NEW BAROMETER. 
435 
The correction for the capillarity of the tube is very slight, and might indeed be 
safely neglected : but it has been considered proper that every source of anomaly, 
however* small, should be pointed out and scrupulously allowed for. The diameter 
of the tube of flint glass is '594 inch, and of the tube of crown glass ’658 inch. The 
correction for these, agreeably to the formula of Laplace, would be respectively 
+ *0048 and + ”0033 : but, in cases where the mercury has been well boiled in the 
tubes, the correction, as found by the formula, should be somewhat diminished. If 
we strike off the last figure in each case, we probably shall not be far from the truth : 
and I have therefore proposed that the correction to be applied should be + ”004 to 
the flint glass, and + ”003 to the crown glass. 
These are all the corrections that, in the case of the present barometer, require to 
be applied in order to ascertain the absolute height at the place where it is now fixed. 
The correction for the height of a barometer above the mean level of the sea, is never 
applied except on especial occasions, and for some definite and express object. The 
formula for such correction, whenever it may be wanted, is as follows * : 
d= + — 
where d denotes the addition (in parts of an inch) to the height of the mercury in the 
barometer, when elevated f feet above the mean level of the sea, in order to show the 
height at which the mercury would stand, provided the barometer were placed at that 
level. So that, assuming the height of the station of the present barometer to be 
9 7 feet above the mean level of the sea (and on this subject I shall have some 
further remarks to make in the sequel), the above expression would become 
■ h 
a — 250-90 + *60 t 
Whence, if the reading of the barometer, at the place where it is now fixed, were ex- 
actly 30 inches, and the temperature 60°, we should have 
30 
d — + 286*9 = + ' 1045 
* This formula is easily deduced from that which I have given in my Astronomical Tables and Formulae , 
page 111, for “ computing the difference in the height of two places by means of the barometer.” For, there 
we have 
/ = a . b . c . log ~ 
b! 
all known quantities except h' . But log— is equal to log b! — log h : and if we make b' = h + d (where d is 
h 
the required difference in the height of the mercury) we have log b' = log h + M . The formula there- 
fore becomes 
whence we obtain 
which is the formula in the text. 
MDCCCXXXVII. 
f = a . b . c . M- 
f-h 
a . b . c . M 
3 L 
