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The standard barometer tube was constructed in 1876, by 
Mr. Hicks, according to the plan of the late John Welsh. 
It has an internal diameter of about 33 millimeters. It 
stands on a solid block of stone, and is supported by a cast- 
iron framework, which was made at the workshop of 
Owens College. 
The catlietometer, with which the height of the barometer 
is read, has a brass scale divided into millimeters, which 
reads to *02 mm. by a vernier. It stands on another block 
of stone, and its distance from the barometer is about 3 ’5 
meters. It was made by Oertling, of London. 
The working barometer is of the form usually known as 
a Portable Standard, constructed by Casella. It is furnished 
with a brass scale divided into both inches and millimeters, 
which reads to '002 inches and • 05 mm. respectively by a 
vernier. It has been verified at the Kew Observatory, and 
reported to be correct to the third place of decimals in 
inches and to have a correction to metrical scale — — '05 mm, 
Preliminary determinations. 
I. Comparison of the catlietometer scale with the standard 
yard, placed at the same distance as the barometer. — The 
standard yard is made of brass, and it has been verified and 
found correct at the Kew Observatory. From the mean of 
seventeen readings it was found that 1000 divisions of the 
catlietometer scale is equal to .999-88 mm. (the probable 
error = + '01 mm.). In the reduction of inches into milli- 
metres 1 apparent inch was assumed to be equal to 25-3916 
apparent mm. when the two scales were at the same 
temperature. 
II. Comparison of the dividing machine with the standard 
yard and the determination of the length of the barometer 
screwhead pointer by the former. — From the mean of ten 
readings its length was found to be =111-340 mm. (the 
probable error =±-001 mm.). 
III. Comparison of the English and metrical scales of the 
working barometer. — If both scales are correct at the 
respective standard temperatures, one apparent inch should 
