Mr. J. Hicks on an Improved Mercurial Barometer. 387 



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 support- 

 ing, 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 con- 

 nexion with a standard barometer, to an air-pump re- 

 ceiver, 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 2/ 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 baro- 

 meters graduated in this way, I found this impracticable, as 

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

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



I have now remedied this defect, and made what I believe 

 is an absolute standard barometer, by graduating the scale 

 from the centre, and reading it off with two verniers to the 

 1 o^ th of an inch. The scale is divided from the centre, 



up and down, into inches, and subdivided into 20ths. 



To ascertain the height of the barometer graduated in 

 this way, take a reading of the upper surface of the column 

 of mercury with the vernier, then of the lower surface in 

 the same way, and the two readings added together will 

 give the exact length of the column of mercury supported 

 in the air, which is the height of the barometer at the time. 



There is another advantage in this manner of graduating 

 over the former, that if a little of the mercury drops out it 

 will give no error, as the column will immediately rise out 

 of the larger tube into the smaller, and become the same 

 length as before ; but by the former scale the barometer 

 would stand too high, until readjusted, which could only 

 be effected by putting the same quantity of mercury in again. 



I have introduced Gay-Lussac's pipette into the centre 

 of the tube, to prevent the possibility of any air passing 

 _«>p into the top. 



2C2 



