14 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



the scale is yio oi yio = yioo of ^^'^ i^^^l^- This principle of matching 

 two scales having spaces of slightly different magnitude is always 

 followed in the construction of verniers, though, of course, the num- 

 ber of spaces embraced by the vernier is varied to suit the circum- 

 stances and the degree of minuteness desired. Moreover, in some 

 instances, the vernier embraces one more space on the scale, instead 

 of one less, than the number of its own subdivisions — that is, 10 spaces 

 on the vernier may be made to correspond to 11 spaces on the scale. 



If, as we have seen, the spaces on the vernier are one-tenth smaller 

 than on the scale, then, in the adjustment shown in figure 3, the first 

 line above the zero on the vernier is one-tenth part of the space, the 

 next line two-tenths, the next three-tenths, etc., distant from the line 

 next above on the scale. When, therefore, we find the vernier in 

 such a position as shown in figure 4, where the fifth line on the ver- 

 nier is coincident with a scale line, it is very clear that the zero line 

 of the vernier must be just five-tenths above the scale line next below. 

 Now, since we imagine these scales to represent inches and tenths, 

 then figure 4 will read, 30.15 inches. 



In many cases it will happen that no single line on the vernier will 

 be exactly coincident with a scale line, but that one line will be a 

 little above while the next line on the vernier will be a little below 

 the corresponding scale lines. 



In the case shown in figure 5 the seventh and the eighth lines on 

 the vernier are each nearly in coincidence, but neither one is exactly 

 so. This indicates that the reading is somewhere between 30.27 and 

 30.28. Moreover, we can clearly see that the eighth line is nearer 

 coincidence than the seventh. We, therefore, estimate that the true 

 reading is about 30.277. We might, probablv, with as great accuracy 

 have selected 30.278. 



If the scale and vernier are accurately graduated, such readings 

 by a practical observer will rarely be in error by more than 0.002 

 inch. It is important in estimating the fractions that the eye be 

 exactly in front of the lines being studied. 



In figures 6 and 7 are shown verniers applied to a barometer 

 scale having 20 parts to the inch. In this case 24 parts on the scale 

 are divided into 25 parts on the vernier. By the principle which has 

 already been explained, the value of the subdivisions affected by 

 such a vernier, or, as it is mostly frequently expressed, the least 

 count of the vernier, will be I/25 of i/4o^%oo of an inch. In reading 

 the vernier, therefore, each line will represent 0.002 inch, so that the 

 fifth, tenth, fifteenth, twentieth, and twenty-fifth lines will repre- 

 sent one, two, three, four, and five hundredths of an inch, respec- 

 tively, and are so numbered. 



As already stated, the lines in this kind of vernier also may not 

 be exactly in coincidence ; but in such a case, owing to the smallness 

 of the spaces, it is not of any special advantage in making our esti 

 mate to consider whether coincidence is nearer one line than the other. 

 In ordinary practice we simply take midway between. Thus in fig- 

 ure 7 the reading is between 30.17r) and 30.178; we therefore adopt 

 30.177 as the proper reading. 



AVhen the zero line of this style of vernier is next above one of the 

 shortest lines on the scale, as was the case in the example above, some 

 attention is necessary in order to take off the correct reading. For 



