180 Scientific Proceedings, Royal Dublin Society. 
accuracy attained is very nearly 0°4 per cent.; or we know 
something of the magnitude in the third place of decimals. For 
purposes of demonstration or teaching, or for the study of the 
important principle involved, this is adequate. 
The apparatus consists of a barometer-tube of thin glass, 
about 100 cms. in length, and about °7 ems. bore, dipping into 
a bath consisting of a small, thin glass flask with two necks. 
A water-tight joint is made by means of a thin rubber collar, 
where the tube enters the flask. Enclosing the bath, and the 
barometer-tube for about 80 cms. of its length, is an outer tube 
of thin glass, 6 or 7 cms. in diameter, which can be filled with 
steam, which enters by a side tubulure below, and escapes by a 
tubulure above. ‘Thus the entire mass of mercury is enclosed in 
the steam-jacket. As the entry of steam into the mercury-bath 
would be objectionable, the bath communicates with the external 
air by a glass tube which enters from the lower end of the 
steam-jacket, and is tightly attached to the second neck of the 
flask constituting the bath. Thus the bath, while raised to the 
temperature of the steam, is directly exposed to the atmospheric 
pressure. 
About 20 ems. of the Torricellian vacuum extends above the 
steam-jacket. The object is to avoid the depression which would 
otherwise arise from the mercurial vapour-tension. If desired, an 
ice-jacket may envelop this upper part, and retain the vapour- 
pressure corresponding to 0° C. 
A few minutes suffice to make the experiment. ‘The heights 
of bath and column are read by a telescope, and to facilitate this 
the water is temporarily dried off the glass surface at the point of 
observation by warming the outer glass with a Bunsen burner. 
Capillarity will cause but a small error if a tube of the 
diameter mentioned is used. A smali correction for variation 
with temperature may be made if desired. As a precaution, if the 
barometric height is fluctuating at the time of observation, it is 
well to note the reading of a reference or standard barometer 
at the moments of observation. or ordinary usage this precau- 
tion is not required. 
The method is available for certain other liquids; and of 
course when the barometric column is long the degree of accuracy 
attained is correspondingly increased. 
