^J4 PORTABLE BAROMETER. 



nny known mountain on the earth, much more so to any height 

 in this country. It will not easily be beUeved by those who 

 have not seen it, that the air will act on a cistern thus com* 

 pletcly closed, and of which the wood in its thinnest partis 

 above a quarter of an inch in thickness : but the fact is, that 

 even when the pores of the box-wood are closed by thick var- 

 nish, except in that part which touches the mahogany tube, 

 in order to prevent the wood from being affected by damps, the 

 mercury on turning up the barometer takes its level almost 

 instantaneously, certainly in less than half a minute ; and that 

 vhen the instrument is suspended by the side of the best moun- 

 tain barometer of Ramsdca's, constructed with an open cistern, 

 no difference whatever can be perceived in their sensibility to 

 the variations of the atmosphere. It is obvious that the variations 

 of altitude in this instrument of dimen^btis above stated, will be 

 one ninety-first part less than in a barometer furnished with an 

 apparatus for bringing the surface of the n:!ercury in the cis- 

 tern to a fixed level : this defect might be remedied by divid-. 

 ing the scale accordingly ; but it is much more convenient to 

 divide the scale to real inches, and make the necessary allow- 

 ance in the result. 

 The barom(;ter The tube and cistern being thus prepared, are mounted in a 

 a mahouaiiv mahogany tube of the size of a common walking-stick. The 

 tube of the size stem of the cistern goes tight into the mahogany tube, and is 

 slick. °' ^^^'^^'^ secured by two small brass screws, or the stem may be 



on the outside cut into a male screw, and so be screwed into 

 the mahogany tube. The cistern forms an head or pummel to 

 the staff when the instrument is inverted for the purpose of 

 being carried in the hand or a carriage. The tube is secured 

 in the mahogany case by passing through perforated corks in 

 the usual way. 



Means and i^or the observation of the height of the mercury, two onpo- 



method ut ob- • ,. . , "^ *^ ^ 



.serving the ^'^<^ '^hts are cut m the mahogany tube, reaching from about 



hci-ht of the 32 to '20 inches, for the longer scales, or from 32 to 25 inches 



for such as are mtended for use in this country. The front slit 



has its sides bevelled, and is exteriorly about threc-f(ftn-ths of 



an inch wide. On one side is fixed a brass plate, divided as 



usual to inches, tenths, and twentieths. On this plate a nonius 



sillies moveable by a small -'xnob, which reads off as in other 



barometers, to the iuOlh of an inch. To this nonius a sma^ 



portion 



