1826.J Measurement of Heights hy One Barometer. 37 



the legs of the tripod ; by the interposition of the body, a pile 

 of turf, stones, &c. ; or by an umbrella properly held by the 

 guide. In gusty weather the oscillations of the mercurial 

 column may be checked by surrounding the cistern end with 

 sods, peats, 8cc. care being taken to preserve the instrument 

 vertical. In tempestuous weather it will sometimes be found 

 impracticable to make use of either tripod or crook. Then let 

 the observer descend the leeward declivity of the mountain until 

 the guide gives notice that the bubble of the telescopic level, 

 pointed at the summit, is stationary in the middle. Mark the 

 precise spot on which you stood, and continue the descent until 

 the eye becomes level with the mark. Then by multiplying the 

 height of the eye by the number of the levellings, the total fall 

 is ascertained ; and here erect the tripod, or fix the crook. 



The summit being an extensive plain, let the observer seat 

 himself on a rock or bank, and holding the barometer firmly 

 between his knees, render it perpendicular to some distant 

 level ridge, or other horizontal plane. The instrument may now 

 be drawn nearer or made to recede, until the mercury stands at 

 the lowest, and then will the column be perpendicular to the 

 horizon. 



When the cistern of the barometer does not coincide in 

 height with the station, but is placed on higher or lower ground, 

 be careful to register a remark to that efl:ect. The difference of 

 level not exceeding five or six feet, one foot answers so nearly 

 to '001 inch of mercury at any pressure, that it will be suffi- 

 ciently correct, and more convenient to add to (or deduct from) 

 the observed height of the barometer -001 inch for every foot it 

 may be suspended above (or below) the station. 



The insUument being securely suspended on the crook or 

 tripod, unscrew the bag as far as it will admit, and tap the sides 

 of the barometer pretty freely. After a lapse of fifteen minutes 

 in cloudy weather; but of double the time when the solar radia- 

 tion is considerable, draw aside the brass tube concealing the 

 interior thermometer, &c. and having noted within a parenthesis 

 ( ) its correct height, place it in the shade fairly exposed to the 

 air, but not to a partial current, at the height of five or six feet 

 from ground that has not been recently exposed to the direct 

 rays of the sun, and does not appear to be exceptionably damp. 

 To find the height of the barometer, keep it steadily vertical 

 with the left hand, and seizing the milled head nut with the right, 

 gradually lower the cylindrical ring, the eye being continued in 

 a line with its under edge in front of, and behind the tube, until 

 the light seen through the narrow slit above the convex summit 

 of the mercurial column is just excluded. Quitting hold of the 

 instrument observe that the contact continues good, or repeat 

 the adjustment. When the instrument is in a narrow glen with 

 dull objects immediately beyoad the slit, fix a sheet of white or 



