De/cription of Humbolt's Barometer. t^Qij 



As the fquare cd cannot then turn, being held faft in the pillar^ 

 the fcrew is loofened, cd finks down floating on the mer- 

 cury, and the atmofpheric air has free communication with 

 the mercury in the tube. The Torricelhan vacuum is formed 

 under /f, and themafsof mercury in the column is increafed. 

 The fcale n o (fig. 4.) is attached to a wooden rod n op, 

 formed of two pieces of timber of a different kind, and 

 fcrewed at p arid q to the mahogany pillar. Turn the 

 three fcrews rrr until the vertical pofition of the inftrument 

 is announced by the plummet s. If you fix the fcale to the 

 tube ka, by fcfewing clofe the fcrews / and m, there will be 

 too much mercury in the pillar. The point of the baro- 

 meter is placed in the aperture of the cock s, and you will 

 have a conjlant level by opening that cock and fuffering the 

 tnercury, which you muft collect in a fmall vafe, to flow 

 down. In order that the ivory cock s may not open of itfelf, 

 there is, at t, ^ fork which by a fcrew can be fixed in each 

 pofition to prevent it. When the abfervation is made, you 

 place in ^ e a fmall cufhion, or covering, held by two fcrews 

 •vv, which prevent the efcape of the mercurj' while you are 

 walking; or you difmount the infl:rument by again puftiing the 

 tube klm (unfcrewcd at m and /} to the bottom of the pillar 

 /", where you turn the fquare cd. The Torricellian vacuum 

 is by thefe means filled, and you turn the tube to the left, 

 holding it with your hand between / and m until you ob- 

 ferve that cd has caught, and that the aperture of the tube 

 is clofed. You then draw out the tube with cd well fhut 

 in order to iuclofc it in its cafe. There is a lofs of a few 

 drops of mercury which adhere to the iron. One of thefe 

 inftruments, conftrufted two years ago, has remained un- 

 hurt during a great many journies over the mod rugged 

 roads and the fteepefl; mountains. More time is required to 

 make an obfervation with it than with a common barome- 

 ter; but it is the only one perhaps which could with cer- 

 tainty be ufed, were it to become deranged even in the deferts- 

 of Thibet. 

 Vol. IV. X Thefe 



