THE SMITHSONIAN INSTITUTION. 



225 



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

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

 the vernier, proceeding from zero, indicates the number 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 the eye, in decimals, the quantity by 

 which the vernier must be lowered to obtain a coincidence, and this is 

 added to the fraction already obtained. This will be hundredths^of 

 millimetres in the metrical barometer, and thousaildths of inches in 

 the English barometers. 



The following figures will serve as an example ; the instrument is 

 an English barometer. 



© 



-10 



— s 



® 

 -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. 7 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 inches. There is then a frac- 

 tion which must be read on the vernier. Seeking which of these di- 

 visions 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 and thirty 

 inches and one-tenth ; no line of the vernier also coincides exactly ; 

 but the line 4 is a little above, the line 5 is a little below one of the 

 lines of the scale ; the fraction falls, then, between seven and eight 

 hundredths. Estimating in tenths the distance the vernier passes 



over between the coincidence of seven and that of eight, we thus ob- 

 tain the tenths of an hundredth, or the thousandths. In this latter 

 15 



