CONSTRUCTION OF A STANDARD BAROMETER, 441 



ACCOUNT OF THE CONSTRUCTION OF A STANDARD BAROMETER, 

 AND DESCRIPTION OF THE APPARATUS AND PROCESSES EM- 

 PLOYED IN THE VERIFICATION OF BAROMETERS AT THE KEW 

 OBSERVATORY. 



BY JOHN WELSH. 

 Communicated to the Royal Society by J. P. Gassiot, esq. 



-Standard Barometer. 



In the course of the years 1853-'54 several attempts were made, 

 under the superintendence 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 one 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 condition 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 the 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, | d as follows: To its upper 



end was attached a capillary tube bent thrice at right angles, having 

 its bore much contracted at the middle point of its length, with a 

 small bulb blown at another part of its length, being finally drawn 

 out to a fine point and there hermetically sealed. To the lower end 

 of the large tube was attached ten inches of a smaller tube, having a 

 bore of three-tenths of an inch, and to that again was added about 

 six inches of capillary tube. , A bulb of three-fourths of an inch was 

 blown at the end of the smaller tube, which, at its junction with the 



