INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 13 



In setting the vernier, the eye of the observer should be brought 

 to the same level as the top of the mercury. A piece of clean white 

 paper placed immediately behind the tube will be found of great 

 assistance in the final adjustment. When observing at night, a 

 strong light should be thrown on this paper. 



The principle of the vernier and the method of reading it. — The 

 vernier is a device by which one is able to ascertaui accurately much 

 smaller fractional subdivisions of a graduated scale than could 

 otherwise be observed by the eye without the aid of a microscope. 

 For example, with a scale having only 20 subdivisions to the inch a 

 vernier enables us to ascertain accurately the one-thousandth part 

 of an inch. The name of the device is derived from its inventor. 

 Pierre Vernier. 



-b 



-30 



30.00 



Figure 3. 



30.15 



Figure 4. 



30.034 



Figure 6. 



4 1_ 



-. 



A vernier consists, essentially, of a small graduated scale, the 

 spaces upon which are just a certain amount smaller or larger than 

 those on the main scale. When t^vvo such scales are placed together 

 some particular li^ie of the one will always be coincident, or very 

 nearly so, with a line on the other, and from this circumstance the 

 position of the zero line of the vernier in reference to the scale can 

 be very accurately determined, as will be readily understood from a 

 study of the following figures and explanation : 



Figure 3 exhibits the manner of graduating a vernier so as to sub- 

 divide the spaces upon the scale into tenths. In the figure, & is the 

 scale and a is the vernier. The lower edge of the vernier, which in 

 this case is also the zero line, is exactly opposite to or coincident with 

 30 on the scale. The tenth line on the vernier is coincident with the 

 ninth line above 30 — that is, a space of 9 divisions on the scale is 

 divided into 10 spaces on the vernier, so that each space on the latter 

 is one-tenth part shorter than a space on the scale. In the present 

 case the spaces on the scale represent inches and tenths; hence the 

 difference between the length of a space on the vernier and one on 



