278 Mr. Newman on the Mountain Barometer. 



screwing up the mercury to the top of the tube, a portion of the 

 metal generally makes its way through the wood, thus soon render- 

 ing the instrument quite useless ; for it is very evident that a baro- 

 meter that loses a portion of mercury from the cistern by making 

 it portable or otherwise after it is adjusted, can no longer be cor- 

 rect or give the height of the column. 



To obviate these inconveniences, I have substituted a cistern of 

 iron in place of the wooden one ; it is fastened to the tube by a 

 thick collar of wood, which is glued on in the usual manner j a 

 screw passes through the centre of the bottom, so as to move in a 

 line with the barometer tube; it is terminated inside the cistern by 

 a piece of cork tied over with leather, so that the instrument 

 being inclined that the tube may be filled with mercury, this cork 

 may be screwed up against the end of the tube, and effectually 

 preserve the metal within from oscillation, without subjecting the 

 cistern itself to any pressure. 



As there is no pressure on the mercury in the cistern, the 

 wooden cap may be left so porous in one part, as to allow of the 

 ready access of air, so that the column shall fall [freely to its 

 proper level, without any danger of losing mercury. 



Another great object in a mountain barometer, is to obtain the 

 temperature of the mercury, which is done by fixing a thermome- 

 ter with the bulb in the cistern ; I have found that by carrying a 

 barometer in my hand and near the body, the temperature is 

 increased considerably, and will frequently rise as high as 85° F. 



In the barometer of common construction, the height of the 

 column of mercury is marked off from another instrument, pre- 

 sumed as a standard, and in that case, the actual height is rarely 

 or ever given, for every change that takes place in the weight of 

 the atmosphere, alters barometers more or less according to the 

 proportion which the diameters of the tubes bear to those of the 

 cisterns, and for that reason, upon examining twenty barometers 

 no two will agree, unless they were marked off together, and 

 happen to stand at that exact height. 



To remedy this source of error each instrument may be reckoned 

 a standard, the height of the column is marked off' from the sur- 



