THE BAROMETER. 



1. THE manner in which a column of mercury, suspended in a 

 glass tube, is balanced by, and measures the weight of, the 

 atmosphere, has been explained in a former number of this series. 

 Such an apparatus, properly mounted and supplied with the 

 accessories which are necessary to indicate the changes of the 

 altitude of the mercurial column, is called a barometer, from two 

 Greek words, p a pos (laros), which signifies weight, and p.erpov 

 (metron], which signifies measure. 



2. To render such instruments generally useful, it is necessary 

 that their indications should be in perfect accordance, that is, that 

 being brought to the same place they should at the same time 

 always have the same altitudes. If this were not the case, obser- 

 vations made with different barometers in different places would 

 not be comparable. In other words, it would not follow that 

 the pressures of the atmosphere in two different places are really 

 different when the barometric columns are unequal, inasmuch as 

 such inequality may arise from an original difference in the 

 materials and construction of the instruments themselves. 



To render the indications of different barometers identical 

 several precautions in their construction are necessary. 



3. It is necessary that the mercury used in the instruments 

 should be absolutely identical in quality, since otherwise columns 

 having equal heights would not necessarily have equal weights, 

 and columns having unequal heights might happen to have equal 

 weights. In fine, the weights of two columns would not be pro- 

 portional to their heights. 



To render the mercury contained in different barometers per- 

 fectly identical, the first requisite is that it be perfectly pure and 

 free from admixture with any other substance. To attain this 

 object is not quite so easy as may at first appear. It frequently 

 happens that small particles of solid impurities, such as dust and 

 dirt, are mixed with mercury ; so much so, that they may be seen 

 upon its surface, often forming a sort of scum. To separate these 

 from it, the mercury is enclosed in a bag of chamois leather, and 

 squeezed there until it passes through the pores of the leather. 

 By this process the chief part of the solid impurities are extricated, 

 since they will not pass through the pores of the leather, and 

 are therefore strained from the mercury. 



Aqueous, and other liquid matter, is also sometimes mixed with 

 the mercury. These are disengaged by boiling it. All such 

 liquid matter is evaporated at a temperature much lower than 

 that at which mercury boils, and they are consequently expelled in 

 the form of vapour long before the mercury reaches its boiling 

 temperature. 



4. When the mercury has been thus purified, it is next 

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