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XXXII. On the Temperature Correction of Siphon Barometers. By 
Wixi1am Swan, Professor of Natural Philosophy in the United 
College of St. Salvator and St. Leonard, St. Andrews*. 
‘a the ‘ Atheneum?’ of the 5th of January, Admiral FitzRoy, 
writing on the subject of the temperature correction of 
siphon barometers, invites attention to an experiment recently 
made by Mr. Negretti. A siphon barometer was heated to 
about 110° from some lower temperature, when it was found 
that, although the mercury rose in the long or vacuum leg of the 
siphon, it did not rise, but seemed to be depressed, in the short, 
or open leg. The late Mr. Robert Bryson of Edinburgh invented 
a self-registering barometer, which is described in the Transac- 
tions of the Royal Society of Edinburgh for 1844+. In that 
instrument, variations in the pressure of the atmosphere were 
indicated by means of a float resting on the surface of the mer- 
cury in the open leg of a siphon tube, precisely as in the ordi- 
nary wheel barometer. Mr. Bryson was anxious to ascertain 
whether his instrument required any notable correction for tem- 
perature; and to settle that point experimentally, a Bunten’s 
barometer was heated to a high temperature. In Bunten’s baro- 
meter the effective height of the mercurial column is ascertained 
by reading two verniers; one indicating the level of the upper, 
and the other that of the lower surface of the mercury in a siphon 
tube. Mr. Alexander Bryson, who made the experiment, found 
that the reading of the upper vernier rapidly changed with increase 
of temperature, while the reading of the lower vernier remained ~ 
sensibly constant,—proving that the level of the mercury in the 
open leg of the siphon was very little affected by change of tem- 
perature. Mr. Bryson having communicated to me the result 
of his experiment, I immediately gave him an investigation of 
the temperature corrections of the two surfaces of the mercury 
in the siphon barometer, of which the following is substantially 
a reproduction. As Admiral FitzRoy has expressed some doubts 
regarding the results of observations of siphon barometers “as 
hitherto obtained,” I have deemed it desirable to make the fol- 
lowing investigation perfectly general, so as to include every form 
of tube; and in the first instance I have avoided employing any 
formula which is only approximately true. 
Let h,, ho be the vertical distances of the upper and lower sur- 
faces of the mercury in a siphon barometer, reckoned from any 
* Communicated by the Author; the results of the investigation having 
been communicated, on the 2nd of February, to the Literary and Philo- 
sophical Society of St. Andrews. 
T Vol. xv. p. 503. 
