[ 507 ] 
XXIII. Account of the Construction of a Standard Barometer, and Description of the 
Apparatus and Processes employed in the Verification of Barometers at the Kew 
Observatory . By John Welsh. Communicated by J. P. Gassiot, Esq., F.R.S., 
Chairman of the Kew Observatory Committee of the British Association. 
Received May 22, — Read June 19, 1856. 
I. Standard Barometer. 
In the course of the years 1853-54 several attempts were made, under the super- 
intendence of the Kew Committee, to prepare, by the usual method of boiling, a 
barometer tube of large dimensions. Mr. Negretti, to whom was entrusted the 
preparation of the tube, succeeded repeatedly in boiling, apparently satisfactorily, 
tubes of fully 1 inch internal diameter. Many of these however broke spontaneously 
before they could be mounted, some of them within a few hours and others after an 
interval of several days. Two or three tubes were ultimately erected, but their con- 
dition was not satisfactory. The adhesion of the mercury to the glass was so great, 
that in a falling barometer the convexity of the top Of the column was destroyed, and 
the surface of the mercury assumed even a concave form. After a few days, rings 
of dirt or other impurity were formed on the glass near the top of the column, which 
soon increased to such a degree as entirely to interfere with observation. The 
mercury employed in filling the tubes had been previously treated for some weeks 
with dilute nitric acid, and afterwards kept in bottles under strong sulphuric acid, 
being well washed with water and dried by repeated filtering before use. Dr. W. A. 
Miller examined specimens of the mercury and could detect no impurity in it. 
Suspecting that some injurious effect might have been produced upon the mercury or 
upon the glass by the great heat to which the tube was necessarily exposed in boiling 
so large a mass of mercury, it occurred to me that the difficulty might be removed 
by another method of filling the tube, which I shall now describe. 
The tube was in the first place prepared as follows. To its upper end was attached 
a capillary tube, ADEF (see Plate XX. fig. 1), bent as in the figure, having its bore 
much contracted at the apex D, with a small bulb blown at E, being finally drawn 
out to a fine point at F and there sealed. To the lower end of the large tube was 
attached 10 inches of a smaller tube, BCG, having a bore of 0*3 inch, and to that again 
was added about 6 inches of capillary tube, GH. A bulb of three-fourths of an inch 
was blown at G, and the smaller tube finally bent into a syphon form at B. The end 
H of the capillary tube was now connected with a good air-pump, and the air very 
slowly extracted, at the same time that the whole tube was strongly heated by passing 
3 x 
MDCGCLVI. 
