958 Newmatis improved Portable Barometer^ S^c. [No. 120. 



The cistern, (shewn in section in the figure) is of iron, and is divided 

 into two chambers, A the superior, and B the inferior ; communicating 

 when in a given relative position with one another by a fine hole o, o.^ 

 passing through the bottom of the upper, and top of the lower chamber, 

 the bearing surfaces of which are ground mercury-tight together ; the 

 lower chamber has motion round the common axis of the cistern 

 through an arc of about 90°, so that by turning it a small quantity only, 

 the connection between the two cTiambers is cut off", the continuity of* 

 the small hole being broken. Into the upper chamber the glass tube 

 is inserted in the usual manner by means of a thick box-wood cap, 

 which is pierced also to admit the stem of a small Thermometer, having 

 its bulb immersed in the mercury of the cistern. A brass cylindrical 

 case in two parts covers the cistern, each part being attached to its 

 corresponding chamber. A hollow mahogany tube is attached by 

 screws to the upper part of the brass case, and a brass scale of about 

 14 inches in length screwed to the wooden tube registers by means of 

 a vernier reading to the 500th part of an inch, (and by estimation to 

 the 1000th part) the height of the mercurial column. 



The instrument having only a partial scale, and this not being direct- 

 ly referable to the surface of the mercury in the cistern, it is evident 

 that to establish a correct Zero or neutral point, the capillarity of the 

 tube must have been accurately determined beforehand ; this done, 

 a comparison is made with a standard instrument, and a point marked 

 on the brass scale, (which is as yet undivided,) gives, after allowing for 

 the difference in the capillary action of the two tubes, the Zero read- 

 ing, or neutral point, of the portable Barometer. 



An example will better explain this operation, upon the accuracy 

 of which the correctness of the instrument depends. 



Let the capillarity of the tube of the standard Barometer be -}-'002 

 inches, and that of the portable instrument -|-037 inches, then if at the 

 time of the comparison for determining the neutral point of the latter 

 instrument, the mercurial column of the standard stood at 25.362 

 inches the corresponding height for the portable Barometer would be 



25.362— (-037— •002)==25-327 inches, 

 which would be the Zero, or neutral, reading at the temperature of 

 comparison ; to and from this point the other divisions of the scale 

 (inches, tenths, and half- tenths,) would be set off. 



