1864.] 



Mr. Hicks on an Improved Barometer. 



169 



III. '^Description of an Improved Mercurial Barometer/' By James 

 HicKSj Esq. Communicated by J. P. Gassiot, F.R.S. Keceived 

 March 16, 1864. 



Having shown this instrument to Mr. Gassiot, he wished me to write a 

 short description of it, which he thought would be of interest to the Royal 

 Society. 



Some time since I constructed an open-scale barometer, with a column 

 of mercury placed in a glass tube hermetically sealed at the top, and per- 

 fectly open at the bottom. The lower half of the tube is of larger bore 

 than that of the upper. 



If a column of mercury, of exactly the length which the atmosphere is 

 able at the time to support, were placed in a tube of glass hermetically 

 sealed at the top, of equal bore from end to end, the mercury would be held 

 in suspension ; but immediately the pressure of the atmosphere increased, 

 the mercury would rise towards the top of the tube, and remain there 

 till, on the pressure decreasing, it would fall towards the bottom, and 

 that portion which the atmosphere was unable to support would drop out. 

 But if the lower half of the tube be made a little larger in the bore 

 than the upper, when the column falls, the upper portion passes out of the 

 smaller part of the tube into the larger, and owing to the greater capacity 

 of the latter, the lower end of the column of mercury does not sink to the 

 same extent as the upper end, and the column thus becomes shorter. The 

 fall will continue until the column is reduced to that length which the 

 atmosphere is capable of supporting, and the scale attached thus registers 

 what is ordinarily termed the height of the barometer. 



From the above description it will be evident that, by merely varying 

 the proportion in the size of the two parts of the tube, a scale of any 

 length can be obtained. For example, if the tubes are very nearly the 

 same size in bore, the column has to pass through a great distance before 

 the necessary compensation takes place, and we obtain a very long scale, 

 say 10 inches, for every 1-inch rise and fall in the ordinary barometer. 

 But if the lower tube is made much larger than the upper, the mercury 

 passing into it quickly compensates, and we obtain a small scale, say from 

 2 to 3 inches, for every inch. To ascertain how many inches this would rise 

 and fall for an ordinary inch of the barometer, I attach it, in connexion with 

 a standard barometer, to an air-pump receiver, and by reducing the pressure 

 in the air-pump I cause the standard barometer to fall, say 1 inch, when the 

 other will fall, say 5 inches ; and so I ascertain the scale for every inch, 

 from 31 to 27 inches. 



It was on this principle that I constructed the open-scale barometer, 

 which has since been extensively used. But having been asked to apply a 

 vernier to one of these barometers graduated in this way, I found this im- 

 practicable, as each varied in length in proportion as the bore of the tube 

 varied, so that every inch was of a different length. 



VOL. XIII. O 



