45, CORNHILL, E.G., AND 122, REGENT STREET, W., LONDON. 



r/H 



c 



h 



a 







differences of "002 inch. The vernier of the figure reading upwards, the lower 

 edge, D, will denote the top of the mercurial column ; and is the zero of the 

 vernier scale. In fig. 6, the zero being in line exactly with 29 inches and five- 

 tenths of the fixed scale, the barometer reading would be 29*500 inches. It will 

 be seen that the vernier line, a, falls short of ^ 



a division of the scale by, as we have ex- 

 plained, -002 inch ; 6, by '004 ; c, by '006 ; 

 d, by "008 ; and the next line by one hun- 

 dredth. If, then, the vernier be moved so as 

 to make a coincide with z, on the scale, it 

 will have moved through '002 inch ; and if 

 1 on the vernier be moved into line with y 

 on the scale, the space measured will be 1 010. 

 Thus, the figures 1, 2, 3, 4, 5 on the vernier 

 measure hundredths, and the intermediate 

 lines even thousandths of an inch. In fig 6*, 

 the zero of the vernier is between 29'65 and 

 2970 on the scale. Passing the eye up the 

 vernier and scale, the second line above 3 is 

 perceived to lie evenly with a line of the 

 scale. This gives '03 and '004 to add to 

 29*65, so that the actual reading is 29'684 

 inches. 



For the ordinary purposes of the 



30 



B 



FIG 



6. 



barometer as a " weather glass," such 

 minute measurement is not required. In 

 household and marine barometers, the scale 

 is only divided to tenths, and the vernier 

 constructed to measure hundredths of an inch. 

 This is done by making the vernier either 9 



FIG 6*. 



or ll-10ths of an inch long, and dividing it into ten equal parts. The lines 

 above the zero line are then numbered from 1 to 10 ; sometimes the alternate 

 divisions only are numbered, the intermediate numbers being very readily 

 inferred. Hence, if the first line of the vernier agrees with 1 on the scale, the 

 next must be out one-tenth of a tenth, or '01 of an inch from agreement with 

 next scale line ; the following vernier line must be "02 out, and so on. Conse- 

 quently, when the vernier is set to the mercurial column, the difference shown 

 by the vernier from the tenth on the scale is the hundredths to be added to the 

 inches and tenths of the scale. 



Price, Standard Barometer (fig 2) 880 



Ditto, with English and Millimetre Scales 990 10 10 

 Ditto, with Tube 0'45-inch internal diameter . 12 12 



