14 BAROMETER. 



value between the two indications must be adopted. An exact 

 observer can never dispense with these verifications. 



e. Nothing more, then, remains than to read the instrument. 

 In the English barometers, the inches and tenths of inches are 

 read directly on the scale, the hundredths and thousandths on 

 the vernier. In the French barometers, with the metrical scale, 

 the centimetres and millimetres are read on the scale, and the 

 fractions of millimetres on the vernier. We begin by reading 

 on the scale the number of inches and tenths of an inch, or of 

 millimetres, there are, as far up as the line which corresponds 

 to the loiver edge of the vernier, and which marks the summit 

 of the column. In the Green barometers, this line marks at the 

 same time the zero of the vernier. If this line does not coincide 

 with one of the divisions of the scale, we read the fraction of 

 the following division on the vernier : — 



The principle of the vernier is very simple. If we wish to 

 obtain tenths, we divide into ten parts a space on the vernier 

 comprising nine parts of the scale {see Fig. 6); each division of 

 the vernier is thus found shorter by a tenth than each division of 

 the scale. Now, if we start from the point where the zero of the 

 vernier and its tenth division coincide exactly with the first and 

 the ninth division of the scale, and if we cause the vernier to 

 move gradually from the ninth to the tenth division of the scale, 

 we shall see the first, the second, the third, and the other divi- 

 sions of the vernier as far as the tenth, coincide successively with 

 one of the divisions of the scale. Now, the divisions of the 

 scale to which those of the vernier correspond, being equal parts, 

 it follows that the space in question has been successively divided 

 into ten parts, or tenths, by these successive coincidences. If 

 the scale bears millimetres, the vernier will give tenths of milli- 

 metres ; if it has tenths of an inch, the vernier will give hun- 

 dredths. By changing the proportions, it may be made to indi- 

 cate by the vernier smaller fractions, as twentieths of millimetres, 

 or five-hundredths of an inch, &c. 



To read the vernier, we must look out for the line that coin- 

 cides with one of the divisions of the scale ; the number of this 

 division of the vernier, proceeding from zero, indicates the num- 

 ber of tenths of millimetres, or of hundredths of an inch, which 

 must be added to the whole number given by the scale. If none 

 of the divisions of the scale coincides exactly, we estimate by 



