52 Descr'iption of the Mountain Barometer, 



ferrule at the end opposite the cistern. This ferrule un- 

 screws^ and shows a steel ring, by which the barometer may 

 be suspended when convenient. 



Along tlie mahogany tube is a scale of feet, carefully 

 divided to inches ; the feet being accurately laid down by 

 small dots, on the heads of brass pins, sunk into the wood. 

 A scale of this kind is alw.iys convenient, and may often be 

 of great use. 



To those travellers whose pursuits may lead them to the 

 measures of the higher class of mountains, I would venture 

 to recommend a barometer constructed with a tube of tv/o 

 feet lonaj onlv : so that the whole instrument should not 

 much exceed t!5 inches in length. This barometer would 

 not of course be useful until the mercury fell below 24 

 inches, which would be at a height of about 6000 feet in 

 the atnxosphere : but its great portability into regions where 

 from both the difficulties of the path, and the rarity of the 

 air breathed, every ounce of incumbrance becomes a serious 

 evil ; and moreover, the great security to the instrument 

 itself arising from its shortness, would, I am persuaded, 

 render it well worth 'while to carry such instruments where 

 great altitudes are to be measured; and it is to be remem- 

 bered that the instrument loses no part of its accuracy when 

 it once comes into action, by being thus shortened. 



Having thus described the instrument, a few practical 

 remarks on the manner of using it may not be superfluous. 



When I am about to make an observatfon, about five 

 minutes before I arrive at the place I take out the thermo- 

 meter, holding it by the upper end at nearly arm's length 

 from my body, and, if the sqn shines, xyi the shade of mv 

 person, fi very soon takes the temperature of the air, and 

 Is not sensibly aff"ected by the heat of the hand. The heat 

 being observed and written down, the barometer is luriied 

 up ; the brass tube half turned ; and the instrument held be- 

 t'.veen the finger and thumb of the left hand aboue the slit, 

 so as to let it hang freely in a perpendicular position. Few 

 persons, if any, have sufficient steadiness of hand to pre- 

 vent little vibrations in the mercury in this position : the 

 hand, therefore, should be either rested against any fixed 



body, 



