224 TENTH ANNUAL RErORT OP 



of the instrument with the left hand, above the attached thermometer, 

 without moving it from the vertical; strike several slight blows in 

 the neighborhood of the top of the column ; then, by means of the screw 

 lower the slide which carries the vernier, until the plane passing 

 through the two lower opposite edges of it is exactly tangent to the 

 sumniit of the meniscus— that is, the convexity which terminates the 

 column. We know that this is the case when, placing the eye ex- 

 actly at the height of the summit of the column, we still see the sum- 

 mit of the column, without there being any trace of light between 

 the summit and the edge of the ring. To convince ourselves that the 

 barometer has remained quite vertical during its operation, we leave 

 it to itself, and, when it is at rest, we look again to see whether the 

 ring has remained tangentical to the summit of the column. If it 

 has not, the verticality had been disturbed ; it must be adjusted anew. 

 It is necessary, at the same time, to examine if the adjustment of the 

 surface of the mercury in the cistern has remained the same. The 

 attached thermometer will also be read anew, and if it indicates a 

 temperature noticeably higher than at the commencement of the obser- 

 vation, a mean valu^'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 tlie inches and tenths of inches are read ■di- 

 rectly on the scale, the hundredths and thousandths on the vernier. 

 In the French barometers, with the metrical scale, the ceptimetres 

 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 whicli corresponds to the lower edge of the vernier, and which 

 marks the summit of the column. In the Grreen barometers this line 

 marks at the same time the zero of the vernier. If this line docs 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 sliorter 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 divisions of the vernier as far as the tenth, coin- 

 cide successively with one of the divisions of the scale. Now, tlie di- 

 visions 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 millime- 

 tres ; if it has tenths of an inch, the vernier will give hundredths. 

 By changing the proportions, it may be made to indicate by the ver- 

 nier smaller fractions, as twentieths of millimetres, or five hundredths 

 of an inch, &c. 



