50 Description of the Mountain Barometer, 



exactly^p half its depth. The bottom of the cistern is closed 

 by a strong lid of box, which screws on the cistern, and, 

 pressing against a leather glued to the inside of the lid, ren- 

 ders the whole perfectly impervious to the mercury in every 

 position. The tube being filled and boiled in the common 

 way, and the instrument held inverted in a perpendicular 

 position, mercury is poured into the cistern till it is filled 

 within two-tenths of an inch of the top. The lid is then 

 firmly screwed on, and secured from being opened, (by idle 

 curiosity,) by a small screw passing through its side. The 

 essential part of the instrument is now finished. The end 

 of the tube, in the cistern, can never be uncovered by the 

 mercury in any possible position, and, of course, no *air can 

 ever enter into it; and, as the areas of the cistern and tube 

 are as the square of the diameters, the diameter of the bore 

 of the tube being -1, its external diameter -3, and the di- 

 ameter of the cistern I'O, the area of the cistern is 100 — 9 91, 

 and there beins; two-tenths of an inch left empty in the cis- 

 tern, the mercury must fall ISS-tenths, or 18 inches and 

 two-tenths, before the cistern is quite full ; a space adequate 

 to the measure of greater heights than any known mountain 

 on the earth, much more to that of any height in this coun- 

 try. It- will not easily be believed, by those who have not 

 seen it, that the air will act on a cistern thus completely 

 closed,- and of which the wood, in its thinnest part, is above 

 a quarter of an inch in thickness ; but the fact is, that even 

 wlien the pores of the box wood are closed by thick varnish 

 (except in that part which touches the mahogany tube,) in 

 order to-prevent the wood from being affected by damp, the 

 mercury, on turning up the barometer, takes its level almost 

 instantaneously, certainly in less than haU" a minute; and 

 that, when the instrument is suspended by the side of the 

 best mountain barometer, ofKamsucn's construction, with 

 an open cistern, no difference whatever can be perceived in 

 theirsensibility to the variations of the atmosphere. It is 

 •obvious that the variations of altitude in this instrument, its 

 dimensions being as above stated, will be one ninety-first 

 part less than, in a barometer furnished with an apparaius for 

 bringing the surface of the mercury-.in the cistern to a C\Ktd 

 C ' .. . level: 



