11 



barometers for measuring great elevations, commence 

 at 15 inches and are carried on to 33 inches. Each 

 inch is divided into ten equal parts, and these parts are 

 subdivided into hundredths by means of a Vernier (so 

 named from Peter Vernier, its inventor) . The Vernier 

 (A, fig. 2 & 3) is a moveable plate, one inch and one- 

 tenth of an inch (together equal to -]-) in length ; these 

 eleven-tenths are divided into ten equal parts, each part 

 being equal to one-tenth of an inch and one-tenth of a 

 tenth, together equal to eleven hundredths. When the 

 pointer of the Vernier coincides with a division of the 

 barometer scale, as in fig. 2, each division of the Vernier 

 will exceed each division of the scale respectively by 1, 2, 

 3, 4, 5, 6, 7, 8, 9, 10 parts, whose denominators are the 

 number of parts between a, b ; the excess of each division 

 being T ^ of a tenth or y l n , T % of a tenth or T ^ T 3 o ot> 

 a tenth or T g^, T ^ of a tenth or T n , &c. The pointer 

 in this position reads off to inches and tenths, viz. thirty 

 inches and one tenth, expressed in figures 30*10 inches. 

 When the pointer does not coincide with a division of 

 the scale as in fig. 3, observe which division of the Ver- 

 nier does coincide ; and the number placed against that 

 division of the Vernier will be the number of hundredths 

 to be added to the inches and tenths. In fig. 3, 7 co- 

 incides with a division of the barometer scale, and there- 

 fore 7 hundredths are to be added to the inches and 

 tenths, and the reading is thirty inches, one tenth and 

 seven hundredths, expressed in figures 30-17 inches. By 

 an alteration in the divisions of the Vernier, the mountain 

 and standard barometer are read off to of an inch. 



