344 
PROFESSOR O. REYNOLDS AND MR. W. H. MOORBY 
The Standard of Tem'perature. 
46. As the most general standard is the difference between the two nhysicallv 
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hxed points of temperature corresponding to the temperature of ice melting under 
the piessuie of the atmosphere and that of water boiling under a pressure correspond¬ 
ing to 760 millims. of ice-cold mercury in the latitude of 45°, taking account of the 
variation of g, the standard in Manchester is the interval between meltino- ice and 
water boiling under a pressure of 760 X 1-0001721 millim. of ice-cold mercury, which 
corresponds to 29-899 inches. And this interval divided by 180 is one degree Fahr. 
According to Regnaults tables a divergence of one thousandth of an inch from 
the boiling point would correspond to an error of 0-0017° Fahr., and this would be 
less than the one-hundred-thousandth part of 180°. 
In order to obtain this degree of accuracy in comparing the pressure of the vapour 
of puie watei, in which thermometers could be placed, with the height of mercury 
over a range of two or three degrees above, and two or three below the point, 
at almost any time, iirespective of what might be the actual pressure of the atmo¬ 
sphere, it was necessary that the barometer, or pressure gauge, while in free com¬ 
munication with the vapour chamber should be shut off from the atmosphere, and at 
the same time so far removed that the temperature of the mercury should not be 
affected by the heat from the gas or boiling water. And, further, althouo’h in direct 
communication with the vapour, this must be such that no moisture could reach the 
mercury ; and, such as involved no current in the passages which might affect the 
relative pressures, as would result by the interposition of a condensing vessel. 
It was also necessary that the arrangements for reading the vertical distances 
between the upper and lower surfaces of the mercury should not only give absolute 
differences of height, but also that they should afford ready means of at any time 
determining the presence of vapour or gas, other than that of mercury, in the upper 
limb of the barometer. 
The Barometer. 
47. To meet these requirements, the barometer shown (Plate 8) was designed. 
The vessel which holds the mercury consists of a bottle-shaped casting of iron, 
3 inches in diameter. Through a stuffing-box in the neck of this, the stem of the 
barometer tube passes. To admit of reading the level of the surface of the mercury 
in the bottle, two parallel plate-glass windows are arranged, ^ inch diameter, having' 
their axis ^ inch from the axis of the bottle. These are sunk into the castinv so as 
O 
to leave the outer cylindrical surface of the bottle clear, the joints between the glass 
and the cast-iron being faced and made tight with a trace of beeswax, the other 
openings into the bottle being one for the admission and abstraction of mercury, 
fitted with a screwed valve, and one for the admission of air, with a mouthpiece for 
the attachment of a tube from the vapour chamber. 
