1841.] Newman's improved Portable Barometer ^ ^c. 959 



The neutral point being thus determined, the true height of the column 

 of mercury above the level of the surface in the cistern for any 

 other reading is deduced in the following manner : — 



Let a b (see the section,) be the line of level corresponding 



to the neutral point, and suppose a fall to have taken place in the 

 mercurial column, a proportional rise will take place in the level of the 

 cistern, and the converse exactly for a rise in the column, which will 

 be accompanied by a corresponding diminution in the level of the 

 mercury below. The variations in height being inversely proportional 

 to the areas of the occupied portion of the tube, and of the cistern ; 

 or, which is the same thing, inversely as the squares of their dia- 

 meters, since, in Mr. Newman's portable instrument, both tube and cis- 

 tern are cylindrical. If then, D represent the interior diameter of the 

 cistern, and d,d' respectively the interior and exterior diameters of the 

 tube, all expressed in terms of the same linear unit ; also if, h represent 

 the height of the column at the neutral reading, and h' any other ob- 

 served reading, then the true height of the column H for that observed 

 reading will be 



d2 



H=h'x(h — h X 1 at the temperature 1° — in which the upper 



sign is to be taken when the observed reading is greater, and the lower 



d2 



sign, when it is less than the neutral height. The factor '^' is con- 



stant for the same Barometer, and is what Mr. Newman calls, the correc- 

 tion for capacity — it is determined experimentally by the maker, and 

 together with the neutral reading and temperature of comparison of the 

 same, is stamped upon an ivory collar attached to the wooden case of 

 each instrument. 



It is much to be regretted, that the ingenuity and care displayed in the 

 construction of the cistern of Newman's portable Barometer, (which 

 is the same in principle as in his standard,) should be entirely thrown 

 away by the exceedingly primitive and imperfect make of the other 

 parts. For reduced, or indeed for good comparative observations, no 

 Barometer is to be trusted, the scale of which is not divided on 

 a brass or other metal rod extending the whole length of the instrument 

 —but this, it has been seen, is not the case in that just described. 

 There would be no difficulty, however, in fulfilling every desired requi- 

 site of the above nature in the construction of Newman's portable Baro- 



