BAROMETER. 



15 



the eye, in decimals, the quantity by which the vernier must be 

 lowered to obtain a coincidence, and this is added to the frac- 

 tion already obtained. This will be hundredths of millimetres 

 in the metrical barometer, and thousandths of inches in the Eng- 

 lish barometers. 



The following figures will serve as an example ; the instrument 

 is an English barometer. 



Fig. 6. 



Fig. 7. 



Fig. 8. 



-JO 



so 



29 







10 





 -.TTo 



In Fig. 6 the regulating line, which is the lower edge of the 

 vernier ring, coincides exactly with the line of thirty inches on 

 the scale. The zero and the tenth division of the vernier are 

 also in exact coincidence ; that is to say, there is no fraction. 

 We shall read then 30.000 inches. 



In Fig. n the regulating line does not fall upon any of the 

 divisions of the scale, but between twenty-nine inches and two- 

 tenths and twenty-nine inches and three-tenths of an inch. There 

 is then a fraction which must be read on the vernier. Seeking 

 which of these divisions coincides with that of the scale, we find 

 that it is the fifth ; we shall write then 29.250 inches. 



In Fig. 8 we see that the height falls between thirty inches 

 2 



