THE BAROMETER. 



the scale of the barometer, extending through three inches, and divided to 

 tenths of an inch. Let C D be the sliding scale of the vernier, equal in length 

 to eleven divisions of the principal scale, and divided into ten equal parts. 



Thus each division of the vernier will be the tenth of eleven divisions of the 

 instrument that is, it will be the tenth part of 1 1 tenths of an inch, but 1 1 

 tenths of an n.^ 'he same as 110 hundredths, and the tenth part of this is 

 1 1 hundredths. THUS it appears that one division on the vernier is in this 

 case the 1 1 hundredth part of an inch. Now, one division on the instrument 

 being a tenth of an inch, or 10 hundredths of an inch, it is evident that a di- 

 vision on the vernier will exceed a division on the instrument by the hundredth 

 part of an inch ; for if we take 10 hundredths from 11 hundredths, the remain- 

 der will be 1 hundredth. Let us suppose that the vernier is placed so that its 

 lowest division, marked 10, shall coincide with the lowest division on the in- 

 strument, marked 28 ; then the first division of the vernier, marked 0, will 

 coincide with the division of the instrument next above the 29th. The divis- 

 ion marked 1 on the vernier will then be a little below the division marked 29 

 on the scale, and the distance between these will be the hundredth of an 

 inch, as already explained. The division marked 2 of the vernier will be a 

 little below the division marked 9 on the scale, and the distance below it will 

 be 2 hundredth parts of an inch, because two divisions of the vernier exceed 

 two divisions of the scale by that amount. In like manner, the division marked 

 3 on the vernier will be below the division marked 8 on the scale by 3 hun- 

 dredths of an inch, and so on. 



Let us suppose that the mercury is observed to stand at a height greater 

 than 29 inches and 5 tenths, but less than 29 inches and 6 tenths. Its level 

 being expressed by the line M, figure 7, let the vernier now be moved on 

 .vile until its highest division exactly coincides with the level of the 

 .. ..ry. On comparing the several divisions of the vernier with those of the 

 .nstrument, let us suppose that we find that the division marked 4 on the ver- 

 nier coincides with that marked 1 on the instrument ; then the distance from 

 the level of the mercury M to the next division below it, marked 5, will be 4 

 hundredth parts of an inch, for the distance of the division marked 3 on the 

 vernier above the division marked 2 on the instrument is 1 hundredth of an 

 inch, because it is the difference between a division of the vernier and a divis- 

 ion of the instrument. Again, the distance of the division of the vernier 

 marked 2, above the division cf the instrument marked 3, is 2 hundredths of 

 an inch, and the distance of the division of the vernier marked 1, above the 

 division of the instrument marked 4, is 3 hundredths of an inch. In like man- 

 ner, the division of the vernier marked is distant from the division of the in- 

 strument marked 5 by 4 hundredths of an inch. This will be manifest by 

 considering what has already been explained. In general, we are to observe 

 what division of the vernier coincides most nearly with any division of the in- 

 strument, and the figure which marks that division, of the vernier will express 

 the number of hundredths of an inch in the distance of the level of the mercury 

 from the next division of the instrument below it. 



The most immediate use of the barometer for scientific purposes is to indi- 

 cate the amount and variation of the atmospheric pressure. These variations 

 being compared with other meteorological phenomena, form the scientific data 

 from which various atmospheric appearances and effects are to be deduced. 



The fluctuations in ths pressure of the atmosphere being observed in con- 

 nexion with changes in the state of the weather, a general correspondence is 

 supposed to prevail between these effects. Hence the baiometer has been 

 called a weath.:r-f,lass Rules are attempted to be established, by which, from 

 the height of the mercury, the coming state of the weather may be predicted ; 



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